To put it plain and simple: They lack any sense of believeability. I know most mecha aren't exactly hard science fiction, but fuck at least they used to try, and some of them got damn close. Now it's like I'm watching super robot shows, where everything's just about how rediculous the escalation can get. One of the things I liked with Argevollen at least is that it feels like it's going back to basics, and I can't help but feel like that's the best way to go.
There's a lot more I could bang on about, but honestly I've banged on about this on every other site I go to pretty hard...I'm kinda tired of doing it. :/
EDIT: I should mention I feel like this is a problem with Sci-Fi anime as a whole, but it goes double for Mecha.
I've never watched Argevollen (actually I haven't watched any mecha series for a long while now, I should really get around to watching that last episode of Unicorn), but I do get what you're saying (I don't talk about anime anywhere but here so I have plenty to say!). My favorite mecha shows are probably still the early years of Gundam and Macross (One reason I'm looking forward to that new Tomino series).
The thing about the term "super robot" though, is that there's actually no such thing. Actually one my problems with what "super robot" shows have become is that they're so self-aware that it's annoying (it usually doesn't ruin shows or anything, it's just really irritating seeing it so much). When people made Mazinger Z or Daimos they weren't saying "let's make this a ridiculous and stupid show", they were saying "let's make a show about some teen with a robot".
On the subject of more realistic mecha though, one thing that bothers me is stuff like the Trans-Am in 00. I don't think it's actually a "believably" problem, it's a storytelling one. They're actually using it "believably" because they're abusing it every chance they get, which is the common sense thing to do if you have an overwhelming weapon like that, but from an out-of-the-universe perspective you shouldn't give the main characters "I win" buttons like that anyway. I've seen a lot of people complain about Kamille's use of the Biosensor in Zeta in retrospect, but that happened in like 2 episodes and the entire show was building up to those moments so it actually felt like a dramatic climax. Character-wise I actually liked a lot of 00 and I wouldn't mind if they made more characters like Setsuna who say a lot without saying much at all.