I have played Avadon and some of the Avernum games.
Avadon is not horrendous as some say, but it's undone by being extremely boring. The story stuff moves between being painful and cliched, and being surprisingly interesting at times. The big problem is simply that it is stuffed full of filler combat, and everything moves at a snail's pace. You are also are forced to constantly re-visit the same areas multiple times over. There is no real free exploration - you go where the game wants you to, and when. The plot will open an area for you or cut it off, often without any good explanation.
After killing your 1000th giant spider in the forest, you'll want to do yourself in as well. Doesn't help that the combat itself is almost devoid of tactics and that the character system is exceptionally simple - you're looking at Diablo-level complexity, if that. All the followers you have map to a class, which wouldn't be that bad except that if you don't like one of their personalities or builds, you can't swap them out with someone else. Not that it even matters too much, because all classes are capable of doing huge damage and have support abilities as well. The hard difficulty is only hard because it resorts to cheap hidden modifiers on everything, so it's not like that makes it much better, just even more tedious.
I have also played some of Avernum, and it's a lot more fun. The earlier games have a very old-school feel to them in that they are extremely open ended and focused heavily on exploration. There is not much in the way of story or deep dialogue and C&C, however, the underground world you are stuck in is creative, somewhat bleak, and interesting to explore. There are tons and tons of secrets and you succeed not by hitting plot checkmarks but by gaining in power enough to delve into the darkest reaches of the abyss. In the later games, this changes to a more typical story setup, however, the writing and C&C are pretty good so this isn't necessarily a bad thing, just different.
I have yet to play Geneforge or Nethergate, but people seem to think they are Spiderweb's best.