The focal decline points of Avadon are its encounter design and story. To be honest, I was enraged by the simplified RPG system at the time (because pretty much no RPG system remains - you choose one of two builds for each class and that's it, you may distribute some points, but it's all superficial since there's only one useful stat for each character and you need to overbuff that stat as heavily as the game allows you), but let's face it - Vogel's RPG systems always sucked. Like, he always offered a shitload of skills in his games (and in Avernum there were also traits), but due to poorly thought implementation there were little freedom there, there were a few right builds and a ton of wrong, useless and impotent ones, not to mention that there were even fewer right ones, so... So yeah, simplified RPG system I can live with, it's unfortunate when instead of healing you amputate, but that's life.
Nevertheless, combat and story are unbearable. After unique (at least amongst cRPGs) Geneforge and semi-unique Avernum (well, there were dungeon worlds before, but Avernum managed to avoid most of the common cliches, no fucking drows and other trash), we are left with boring power struggles (boring since each side consists of bad, ugly motherfuckers and you're given no reason why you should really prefer one side for another) in a 100% generic dork fantasy world. Yay. Oh, and faux C&C - double yay!
But even that could be tolerable - after all, despite decent setting, the story in Avernums was always DERP DERP DERP, always this shit about good and decent and brilliant avernites vs evil empire, but it was the excellent (at least in parts 5 and 6, hadn't played the other ones) combat that kept you from uninstalling the game - engagements were diverse and challenging, while in avadon we see what? First, uninspired, boring design where most of your enemies are just copy-pasted grind filler, most bosses are copied from Vogel's previous games (in worst and unimaginative manner possible) and new battles are simply sleep inducing (like clusterfuck that the final boss is, I mean, it's probably in eternal top ten of worst game bosses ever). Oh, and hard is the new easy, obviously.
So, basically, Avadon is awful not because it became too streamlined (no real C&C and simple role-playing system), but because it's gameplay is shoddily crafted, with spending as little effort as possible (it shows, really). A hack job, really.