Increasing a user's home directory size, external drive? My mum has an iMac with a 3TB fusion drive in it and she's running out of space. She loves photo and video editing. She's good with final cut but not with managing separate storage. SSD's do not go up to that size and if they did, they'd be massively expensive. The fusion drive still uses regular storage under the hood for the majority of it as far as I can tell. 

Would it be a good idea to move her home directory to a fast external hard drive? Say I get her a thunderbolt 3 capable hard drive with 8TB or 12TB, like the LaCie's. She doesn't have to try and manage several drives for her work, but she does have a ton of space afterwards. Are there any performance concerns or upgrade issues I should be aware of?
Moving the entire user domain, instead of just the folders for Videos and Photos may be overkill, though it would keep things simple. You're not going to see any advantage from moving preferences, audio files, Word docs, etc, as they are too small and not causing any bottleneck.

There's no disadvantage either, except that if the Mac boots up and can't find the drive, it will create a brand new (empty) user account on the system disk. "Where has all my stuff gone?" 

Don't forget that you'll need another device to back up to!

Fusion drives can reduce in performance as they fill. I would recommend swapping one or more parts out for a regular SSD in due course.