As far as completion: I managed to 100% Mirage in 50 hours, and part of that was spent aimlessly exploring. I’m sure it could be done faster. Of course, it’s also the weakest entry in the series in a while, and the easiest to skip.
I feel you on Valhalla. 300+ hours and I still had side stuff unfinished.
E.T.
Yes really. I played it all the time as a kid and didn’t think it was any more difficult or abstract than the rest of the 2600’s catalogue. Granted, we kept the manual, which made a huge difference in understanding and enjoying its bizarre logic, but still. I had no idea it was so hated until at least a decade later.