Let's see... Gothic series... Risen... Yeah, with that high a pedigree, I can't imagine what went wrong.
Except the Gothic games are some of people's favorite games...sooo...
Piranha Bytes' biggest problem is they really haven't evolved their games much w/ each game.
When Gothic launched, RPG's with full-blown voice-acting & immersive game-world w/ NPC's on schedules doing stuff (NPC's sitting in camps, NPC's talking to each each other, blacksmiths grinding things, blacksmiths hammering stuff, etc etc) was relatively new. It's not like Gothic series had the best voice-acting back then either; it was a new thing, so it was often given a "pass" b/c it was cool to have a whole RPG w/ ridiculous amounts of words being voice-acted. When other games start doing the full voice-acting thing and have great voice-acting (i.e. go see KOTOR and any BioWare games after that) - yeah, what PB's doing is basically "more of the same" when they don't have fantastic voice-actors everywhere.
With each release, PB has experimented and tweaked certain things a bit to their games (i.e. Gothic 3 added mouse-look for combat and the game-world become way more open-world and more ARPG with one-off quests everywhere; Risen 2 changed the camera so you can make it be able to lock and unlock which majorly changed the combat feel) - but never really much to really radically overhaul or change things to their core RPG style.
To me, Gothic 1 is probably their best game...and honestly, I don't think they'll ever top that. And honestly, their second best game is Risen 1, which is an awful similar in design to G1 in many ways.