Black Isle is why BG 1 and 2 were good.
Common misconception. BG1 was 100% Bioware, Black Isle helped with BG2 but it wasn't gamechanging help.
BG1 and BG2 is almost 100% child of Bioware.
Problem is it looks like they were good enginers creating rather good story from rich lore of D&D.
NWN was fuckup in therms of game-play, mainly because D&D3 was clausterfuck to recreate in a game and it was one of the first batch of full 3D games so there was no fluid and it was super clunky in comparition to BG2. Storywise it was alright with excellent second expansion.
When they dropped D&D, they started to write their own lore. Aaand that's when problems started.
Their writers SUCK. Period
ME has pot holes everywhere.
KOTOR1 evil characters have cartoon quality dialogs.
Jade Empire story wasn't exciting.
DA had some nice lore but actuall story and dialogs... were terribad.
In therm of gameplay it depends. ME3 and 2 had good gameplay few good set pieces but actual story in both games were... uhh.
EA is not a problem. Bioware writters mostly suck.
They have many good values like character design (Illusive Man) or level design (Mass Efect 3) but cores of RPG which is story, dialogs and actual rolaplaying is cringe worthy.
If i would be in Bioware i would do everything to buy Obsidian for their writters and designers.
edit:
Oh i forgot to mention. In time when they were developing Mass Efect someone probably hired some
HArlequin writters to achieve those cringe worthy romances.