I dunno'. In any military shooter, what variety is there? You kill dudes with guns. I mean, in any game that's at all realistic, you kill people with guns. Or maybe people with knives. I mean, there's not a lot else to go on unless you start adding monsters or whatever to the mix. They probably could have added a few more enemy types to the game, but I really don't think it would have made a ton of difference. It would have just been another dude with another gun/plasmid/spell/whatever. Granted, a few more wouldn't have hurt, if only for the *mental* effects of having variety, mechanical effects aside, but it still didn't bother me all that much.
I don't agree that the shock+wrench combo is the best, though. Sure, it's great if you get all the upgrades for those, plus all the others that go well with it. Like the armor upgrades and the defensive shock upgrades. But if you sit down and work out some other builds, there are plenty of options which are just as effective, including some that combo better with guns instead of the wrench, and others that work much better for larger groups, and others that work better against groups of different kinds of enemies. I've played through the game a number of times now, and I think there's plenty on the table in that regard. Sometimes the combinations just aren't as obvious since the game doesn't spell any of them out for you other than the initial shock+wrench combo, which I think was a mistake... they shouldn't have put "hints" anywhere in the game about plasmid use, not even in the beginning for tutorial purposes. We pander to people too fucking much these days, and those (shock+wrench, using shock on water, lighting oil on fire, etc.) should have been discovered by the player, not dropped in their lap. But seriously, there are a lot of other options available if you get creative. Toward the end of my last game, I didn't even fire weapons at all. My setup was more or less entirely defensive with a few alternative options in case things went south.