We do pretty much everything ourselves, and over the years have rebuilt engines, repainted top to bottom (including a rig), modified interiors, and added/installed any new purchases with a few exceptions.
Things I'd tend to leave to the pros:
Rigging work unless it's a simple stay/shroud replacement (which I'd get professionally made up)
Canvas work other than simple covers
I probably wouldn't tackle a transmission rebuild.
Maintaining and doing as much work on your own boat is, to my mind, a large part of the enjoyment of it all, with the added bonus that when things inevitably to wrong somewhere, you're familiar enough with things to deal with it yourself.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)