I suppose compatibility's an ongoing issue for a lot of people.
I've been quite lucky - Of course I looked into compatibility in advance to make sure I'd be OK but, as it happens, everything I use just works fine.
I don't use a huge amount of plugins + vsts...Waves platinum, SSD5, Drumcore, EastWest platinum orchestra and ProTools.
That's about it and, thankfully, nothing needed updating except ProTools, which I was always going to have to do anyway.
Strangely for audio purposes everything's been super smooth sailing - My Steam gaming library is where the real damage was.
There's a huge number of older titles which will probably never be updated for MacOS 64 bit only.
That's a shame but not a surprise.
Edit : Actually, I lost Soundflower - Forgot about that.
For anyone who relied on Soundflower for anything, check out Blackhole. I've been using it as a replacement without issue.