Huh? Max bomb load for an 88 is 4 x 500kg, right? That's about 4400 lbs., the B-17 carries 6,000. And rugged? Maybe compared to a Zeke! Not very tough compared to a B-26 or any of the 4-engine jobs, though, and when you add in the popguns it makes easy meat for fighters. And as Lusche pointed put it climbs like a wheezing hippo (faster than fully loaded Lancs, maybe, but Lancs carry three times the bomb load).
It has its uses (it's great for torpedoing CVs whose owners have foolishly neglected to take out ords at the field they're attacking) but I think there are good reasons you don't see it much in general use in the MA.
Add up the "20 - 50kgs". Unlike this game, B-17's weren't "dive bombers". The Ju-88 on the other hand, could. So you could surmise that in order to sometimes handle the additional stress of dive-bombing, that yes they were rugged.
Lancasters and 17's are "Heavy Bombers".
You're comparing "Heavy Bombers" to something that is similar to a "Mosquito/A-20" and can still tote 5409lbs. I use the 88's and typically sink CV's with a single formation.
As for defensive armament. Again, the 88 isn't a heavy bomber, nor a medium bomber.
