Internal or external SSD with system files?

My current 2011 iMac has an internal hard drive and also an SSD in the old optical drive space with system and apps. It’s a great setup which starts and runs nice and fast.

As the 2011 iMacs don’t support Mojave I’m after a newer used iMac and have been assuming I would use the internal hard drive for data storage and an external SSD for system and apps. I like to store all files locally (in addition to iCloud drive and backup).

However I’ve seen an iMac for sale with just an 256 internal SSD which would necessitate an external hard drive for data storage.

Can anyone think of disadvantages of this option - smaller internal SSD for system, external hard drive for data?

I run this way all the time. Small ssd inside, large ssd outside.
There is absolutely nothing wrong with that.

I agree with @JKoopmans. My first MacBook Air only had a 128GB SSD. It held system and apps and just those files I might need when mobile. I kept the bulk of my files on a spinning 1TB usb drive.

I have a 2011 Mac Mini which I use headless. If I replace it with a new one, I’ll get the base 128 GB model and pair it with a large external drive. Most of my files are static so, in my case, I see no advantage of a larger internal SSD.

I guess one issue is that you would need a cloud backup solution that backs up external drives.

[Heads off to Google ‘does Time Machine backup external drives’…]

Yes it does