I've had two crashes in the 5 or so hours I've put into the game- one before I installed Buffout, and one when i tried to fast travel to the train crash. Otherwise it's running about the same as Fallout 4 for me. There's definitely some bugs and optimization issues, but on the whole I'm pretty happy thus far.
A couple of things that have helped with bugs/performance:
- I've got the GOG version of F4 as a separate install rather than mucking about with the downgrader
- Dropped my settings to High instead of Ultra, dropped shadow distance to Medium
- If you've got two installs, they'll both create "Fallout4" folders in My Games. Rename the Steam one to "Fallout4 Commonwealth" or "Fallout4 Steam" or the like when you're not using the steam install, so London doesn't try to read the wrong .ini file
- Install Buffout, it's a bundle of engine fixes recommended by the devs. Just drag and drop it into the F4SE folder of your London install.