For me it’s Adobe. Stupid government and other forms I need for work. No available third party software will work on some of them, and I’ve tried them all. And for some dumb reason even having an Acrobat subscription won’t let me fill them out on their (extremely buggy, by the way) web app.

Was never able to get it working with wine.

That, and the compatibility between Libreoffice (or other Linux office software) and MS Word is still terrible. I’m quite sure that’s mostly due to shenanigans by MS. So if you have to swap documents back and forth between people (e.g. with track changes) it just doesn’t work as well. And, like Adobe, the web app version of Word seems to be kneecapped in a few key places.