There's nothing wrong with a direct impingement system. As long as you don't mind really cleaning a rifle every 1000 rounds or so, there shouldn't be an issue. I fire the cheapest brass-cased stuff through mine and have had no problems.That's the thing that really annoys me about those Keltec SU model .223 rifles. I REALLY wish they would make one at a similar price point, that consisted of the same basic operating mechanisms and components BUT, in more of an AR layout with a pistol grip, stock, etc. It would be a nice and inexpensive alternative in .223 for avoiding the crappy direct impingement system of AR's....for someone lazy like myself that doesn't want to deal with cleaning a DI system.
For cheapo .223 alternatives to the AR there is also Mini 14's and the newer production AR-180B's (I think that's the proper nomenclature). Most of those you can usually find at least a few in the $500-700 range sometimes.