Well, I've ran across a bunch in my time. I remember Living Steel as being unfun to learn.
But the most complex was a system ... and the name escapes me .... but I recall being interested in it (this is the 80s), and it was modern warfare-style, but the rules for combat with the weaponry, and the amount of weaponry, were just .... oof. Ballistics ain't easy when you're making them super-realistic.
I'd imagine that trying to make modern warfare realistic would result in it being deadly. Not necessarily a bug, mind, just not necessarily a feature, either.
I used to have a gaming buddy who was a collector, mainly in the 1980s and 1990s, and he had a lot of really weird stuff that I have no idea what it was called or whether he was running it exactly right.