My replies to the issue of not using unofficial classes are the following:
Volo's is all optional. When I ran 2nd edition the only thing I allowed in the game was core classes from the phb. No splat books etc. A lot of the stuff from volos guide came from unearthed arcana, some of which was probably battle tested, some possibly not. The gist of my reply is "it honestly depends on what the dm wants to run". As well the DM should have a discussion with his players. It's hard to keep up as a DM, and there sometimes is issues with the non official stuff. But really its about knowing exactly what the class is capable of.
as an example I really want to run some Al Qadim. I will probably rule the following: Traditional races only (Humans, Elves, Dwarves, Halfings). Core 4 classes. Plus optional flavour classes from the DMs guild. No warlocks, sorcerers etc. And no cantrips.
Unlike traditional fantasy worlds (forgotten realms) Ravenloft is a creepy assed place. It's mysterious, and it's not normal (traditional fantasy). There is a very real possibilty of becoming the Undead, A Vampire or a werewolf. Starting off the game as already undead takes a bit of the creep factor away. Starting off as a able bodied fighter who isn't scared of a little orc blood on his sword, but is very scared of the undead is a bit more plausible. It's got a call of Cthulhu kind of vibe to it. Basically its friggin great.
My suggestion to the guy was the following: