I wouldn’t think Mail would use iCloud, other than to sync credentials and account information. But like you I can’t recall having problems with low volume apps like Calendar, Reminders, Drafts, etc.
iCloud data is stored on non-Apple services that don’t have these kinds of problems. This makes me wonder if Apple has a choke point in their network? Seems like that should be easier to fix than adding storage capacity.
I believe Mail.app uses iCloud for .iCloud, .me, and .mac email accounts.
I may have asked this before but can never remember the answer: does Apple Notes allow searching within the text of PDFs? Most of what I’ve used Evernote for over the years is as a repository for my scanned documents (receipts, invoices, contracts, etc.). The search feature in Evernote is, for me, the killer feature. If Apple Notes can do that, however, I’d love to pay for one less tool.
Not to mention that I now use the Web interface for Evernote on my Mac because I don’t want to install their “native” application.
Yes it does.
I extensively use handwritten notes using the Apple pencil on my iPad, and it makes a good attempt to decypher my terrible handwriting.
My need for handwritten notes limits me to some extent - I can’t use SimpleNote or Obsidian, for example. It also makes it a bit more challenging to back up - for really important notes I want to be sure to keep I print to PDF on my Mac and save them in my Eaglefiler repository.
I’ve had a minor delay in syncing notes at one point, but so far it’s working well enough for my purposes. My biggest concern is the lack of incremental versioning (or backup of non-text content). If I delete a paragraph or some text from a long note, there’s no way back as far as I can see.
True. What I meant was iCloud mail is a standard IMAP system. Since our clients connect directly to the email server they wouldn’t be subject to problems with iCloud storage.
Update to this: Apple Notes sync is terrible! - #30 by MereCivilian
My sync issue has been resolved by itself. Strange but I will take it. hopefully, this was a once off 
I gave up trying to use Apple Notes for all my stuff.
Reasons:
iCloud sync was unreliable, as you already experienced. My 22k notes never seemed to stop syncing and between my two Macs and the iPhone and iPad the note numbers never matched. I believe the initial sync confuses iCloud and, from what I felt, syncing is never performed in background. So when you are out and about and try to reach that note you need, you find that Apple Notes starts merrily syncing all the stuff it believes it is missing. This gets aggravated with shared Notes folders with my SO. We never had that kind of experience with Evernote, so I am still using it for shared notes. To sum it up, I do not trust Apple Notes sync via CloudKit.
The lesson, coming as an Evernote refugee (still a subscriber, though, here’s still hoping for an EN strong comeback!): never use anything that does not store everything on index files. I am now using DEVONthink, but also considering some more lightweight alternatives like EagleFiler, Notebooks or KeepIt. But in the end I may end up resorting to storing everything on iCloud and be done with it.