The DOOM engine isn't the problem regarding physics, it's how it's setup. It could be tweaked to have better physics. But regardless of some of these problems, I have always been impressed by SRB2. I think people tend to forget where it came from. It's come a long ways, and for getting it where it is I think the team has done a wonderful job. It's too easy to look at where you want it to be rather than what it's taken to get where it is right now. Don't overlook that.