This.
You’ll probably be fine. But if you aren’t, expect pain.
Mine lasted 843 days. Can you tell that number is burned in my brain? I wrote about the saga here.
My favourite part of all that was learning that even Apple don’t know how iCloud works. Seriously.
It’s like commuting daily on a well developed city highway system but there is a small chance you will end up stranded in the middle of the Nullarbor and out of fuel.
I’m pretty sure that there are people in Apple who do know exactly how iCloud works, have a list of “known issues” and risks and probably have a comprehensive list of tips for reliability and recovery they use themselves. Part of the issue is Apple’s culture that people who know anything about anything are wrapped in such tight security that they are not allowed to share any of that knowledge with anyone else in Apple, and especially not with anyone in the support team, let alone with any customer who might really benefit from it.
At one point a senior support person told me that engineering had told him on the phone that they knew what the problem with my iCloud Drive was, but they’d used a term which was proprietary to Apple, and so he was not even allowed to tell me what it was in simple terms. I just had to accept that it was broken, that engineering was working on it and it would (at that point) take “weeks to months” for them to fix it.
I use iCloud Drive exclusively and it is rock solid. I have OneDrive and DropBox, but don’t use them. However, I use exclusively Macs, I don’t use a PC. A couple of years ago I was using iCloud Drive on a PC and it was a complete mess. Didn’t work. But on my Macs/iPad/iPhone it works perfectly.
Apple released new Windows apps for iCloud stuff recently. Still does not work great. OneDrive on the other hand works great on a Mac. So if cross platform is needed, do not use iCloud unless you are good with the web version on Windows.
Amazon, Google, Microsoft, and many other cloud providers publish Service Level Agreements which state the Uptime Percentage that they agree to provide. For example “. . . the Monthly Uptime Percentage will be at least 99.9% in any calendar month . . .”
Welcome to iCloud is the closest thing to an SLA I’ve been able to find.
iCloud reliability has been discussed here many times and has been described as everything from "rock solid’ to “can’t be trusted”. I have no reason to doubt any of these statements.
“iCloud” is a many-headed beast that has to deal globally with billions of of users, devices, and data centers. Not to mention that the basic concept of real-time sync, of great varieties of data, held globally, is hard to implement (and that is a real understatment. 
) On top of that, we all use iCloud differently, with different amounts of understanding and expectation. I’m amazed it works as well as it does.
And yet I don’t see many people complain about DropBox and OneDrive (and there are probably a lot more users of OneDrive). They both just work (but have their own issues).
I used Dropbox for years and loved it early days on Mac and iPhone (personal use) but moved to iCloud (and particularly CloudKit) as it got better and Dropbox got more intrusive on my Mac. (For example, Why so much criticism of Dropbox? - Software - MPU Talk.)
I coded, trained, and wrote documentation for software on Windows PCs and Unix backends for my entire career. And I can’t tell you how happy I was to be off the Microsoft platform since I’ve retired.
… And yet (you say) I don’t see many people complain about DropBox and OneDrive …
In my experience, iCloud Drive as a Dropbox alternative works well. I have even found that the aggressive “disk optimisation” techniques that Apple use to avoid the user to selective select which files need to be downloaded works fine with the tools I use (Finder, Obsidian, even DEVONthink is aware when a indexed folder or file is not present and requests the download). But this may vary.
I am quite happy with iCloud Drive but I suspect that Apple is using the same frameworks that have been published for File Providers on Mac land all the usual vendors (OneDrive, Google Drive, Dropbox) should work equally fine, UX-wise. Not so sure if they are as opaque when something goes wrong as Apple is (I have been lucky).
The only thing I dislike is that the network filesystem mount point is hidden somewhere inside $HOME/Library/MobileDocs or something like that so it’s a nuisance on the Terminal.
That wasn’t my point, I said they have their problems, but reliability is not one of them. Dropbox’s app have gone south, but the one thing I have always heard it is that it just works.
OneDrive also has it’s issues, but I have been using it on a corporate level for years (as well as on my home Macs and PCs), it has never lost a file or just stopped syncing for some mysterious reason.
And heck, iCloud has been very reliable for me for years now, but I also don’t use it for as much stuff and it is noticeably slower than OneDrive.
You mean, perhaps, “Hard for Apple”. sync has been solved by most, if not all of the other cloud service providers - Amazon, Google, IBM, Microsoft, and even Dropbox.
You are assuming an always-Internet connected user. I am often mobile at client sites for periods of time with no Internet connection - I cannot rely on a algorithm deciding which files can or cannot be available on my computer as there is no faillback when it makes the wrong choice and I have no connectivity at that time.
FWIW, I am using iCloud Drive exclusively now and I have not had any more issues with it compared to OneDrive. In fact, I find it more solid than OneDrive at this time (still need to use it for the work laptop).
I left Dropbox many years ago, so don’t have any current experience there.
I find iCloud Drive very practical when working interchangeably between my Mac and iPad, and the occasional access from the iPhone. Granted, I don’t have much travel, and when I do, I’m always in range of a 4G/5G cell tower.
Guess I’ll just have to take your word for that, eh?
Yes, you are absolutely correct! Of course, there is the Finder “Download Now” that you could use, but first, you need to remember to download the folders you would use (not always possible) and second, you need to trust iCloud Drive not to evict those downloaded files on its own before they are needed while offline.
That exactly my point - Dropbox “selective sync” is simply a setting that says “I always want this folder stored locally on this computer” or “I never want this folder stored locally on this computer”.
IMHO, that’s a lot SIMPLER than the smoke and mirrors, black box, “you don’t need to know” way that iCloud works.
I think Apple’s philosophy of trying to keep things simpler is actual an epic fail here. It is simpler for me to understand what I want is or is not here rather than “it might be here” depending on how some wizard behind the curtain decides for me?
No need. I purposely used the words “most”. I think that covers me 
To be fair, I haven’t used Dropbox or OneDrive in 5+ years but I don’t recall ever waiting several minutes or longer for a sync to start. Google Drive starts to sync as soon as I save a file, and I’ve seen changes I’ve made on my Mac appear almost immediately on my iPad. That’s probably an advantage of using a web based app.
I don’t know why iCloud is frequently slow to begin syncing apps and files. Apple stores data on both AWS and Google Drive but iCloud has never matched their upload speeds.
I think there are people who know how iCloud should work. I’m not convinced they know how it doesn’t work.
But, as you say, those who know most probably aren’t saddled with looking at problems. I would have dearly loved to have heard from someone who could tell me what an ‘iCloud reset’ actually did. Multiple senior advisors and ‘engineering’ could not tell me anything more than trite generalisations and when I fired questions back the response was usually ‘Oh, good point. We don’t know.’
Oh, iCloud is up. It’s just not doing what you expect. Like a car that reliably starts, but the steering wheel occasionally disconnects from the steering rack.