• Signup can be tricky.
• Managing your encryption keys is tricky for normal people (I know someone who signed up for it on incognito because they weren't sure about it yet then got a bunch of confusing popups when they signed on with their phone).
• Room organization is missing a layer used on discord (server->room instead of list of rooms) leading to confusing moderation structures and nearly required manual organization of rooms if you're in more than 10.
• notifications rules can be obtuse.
• having different commands based on the clients used can lead to confusion.
• most clients have security related popups that just confuse people (This person reset their identity!).
• people can struggle with how to properly interact over federation, much like in the fediverse
• screensharing tools just aren't there yet

Things have been getting better fast for matrix, but its just not ready for the masses IMO. I still suggest it when I can when the use case makes sense.