I am saying that they were all fairly ridiculous, in their own ways. ME1 had Tali's silly voice recording and a Reaper plan that made no sense, apparently put in with some vague expectation that they'd figure out a way to explain the plan in a later game. (If they had actually ever planned to make the Reapers truly irrational and incomprehensible, the latter could have been tolerable, maybe. But they never intended any such thing.) ME2 had Shepard's idiotic and pointless death and resurrection, a Collector plan that had no point, and a big Reaper reveal that actually managed to make the ME1 Reaper plan even less sensible than it already was. These are just the ones that I feel like typing. I could do more from each, plus a bunch from ME3, but you get the drift by now.
Most of these problems don't carry over into the next game. But the incoherent Reaper plan is a problem for ME3, because ME3 is the game where such things needed to be resolved. ME1 and ME2 threw out some pretentious mumbo-jumbo and left the ME3 writers to sort it out. It is neither fair nor rational to blame ME3's writers for story problems that were introduced in the previous games.
The writing and everything you mentioned in ME1 and ME2 were absolutely fine and made sense for the most part (the game has a little bit of space magic, but so what, it's fiction and isn't exaggerated out of proportion). The collectors had a point, i.e. build a Reaper to open the mass effect relay in the Citadel. Shepard's death and resurrection was fine, space magic tech FTW. No idea what Drew has to do with Tali's voice acting. The Reapers did not have a plan that made no sense, their plans and agenda were simply not revealed, other than the fact that they wanted to destroy/enslave all intelligent, spacefaring life in the galaxy, which made them very mysterious. Sounds to me like you just simply have a problem with this type of space opera fiction.
I have no idea why you are blaming Drew Karpyshyn for the ridiculous and terrible writing that Mac Walters was responsible for in ME3. The hole that you mentioned, the one ME3 fell into, was only dug in Mass Effect 3 by Mac Walters himself, it was not dug by Drew Karpyshyn in ME1 and ME2. You also seem to completely renounce the fact that nobody really had any problems with the writing in the Mass Effect series until ME3 was released.
- The Reapers were truly irrational and incomprehensible until Mass Effect 3,
- The force that drives the Reapers, which is to kill all organics so that they don't kill themselves (completely ridiculous and illogical, lol), was not mentioned until Mass Effect 3
- Star Child, "god" of the galaxy, was never mentioned until Mass Effect 3,
- that Shepard must pointlessy sacrifice himself, in order to prove that humans deserve to survive (what? lol), which ultimately ends the reaping cycles, was never mentioned until Mass Effect 3
and countless other ridiculous story elements, that were developed during the production of Mass Effect 3. There is absolutely no connection of these story elements to the story elements in ME1 and ME2, other than the fact, that the Reapers exist and have an agenda.
Not to mention that you bash Drew for the discarded ideas that the writing team had developed and are trying use this is proof that Drew is a terrible writer, although Drew himself even mentioned that "Some of the ideas were a little bit wacky and a little bit crazy". What? Seriously?