The big one for me is the Level 4 vendors. They're basically all buggy. The Scribe, Smiling Larry, Ron Staples, and Doc Andersen won't stay at my settlements. I've randomly encountered all of them and successfully recruited them, and they either never show up or disappear shortly after I assign them. I've tried moving them back with the console, and they just end up disappearing again.
Anne Hargraves, Trader Rylee, and the Vault-Tec Rep all stay at the settlement, but they're still buggy. Their dialog is like caught between being recruited and not being recruited. I still get a dialog option to ask Trader Rylee to come work for me, despite her standing at one of my vending stands and already working for me. The Vault-Tec Rep just gets stuck in this dialog loop and I have to keep working through all of his scripted lines before I can barter with him.
Holt Combes from Vault 81 is supposed to be recruitable as a vendor, but the dialog with his wife is broken so you never get the opportunity. That's another one I'd love to see fixed.
Tina De Luca is recruitable, but bugs out unless you make her brother turn violent toward you during the Dependency quest. Even after recruiting her, she never talks. Although I actually heard her talk for the first time today, oddly enough. Her dialog made it sound like she was still back in Vault 81 though.
Curie bugs out for a lot of people and won't go to their settlements. A notification saying that she's going there will pop up, but she'll just go back to Vault 81. I managed to find a workaround for this, but it's a bug that a lot of people are experiencing. I'd consider this a fairly major bug, considering that Curie is one of the most popular companions in the game.