Sayaka doesn't tend to panic (recklessness when fighting solo like every time on-screen on the other hand...) and in canon was pretty much completely isolated. What wouldn't happen would be us letting them go off solo without advice or trying to nip problems in the bud. And a dumb hero complex? From our standpoint, that's pretty much desirable- and honestly, Madoka wasn't so different on that front.
Again, canon was pretty much tailor mode to give Sayaka enough rope to hang herself with. Grief seeds became a question of morality which ended badly thanks to a certain rat's intervention, she had just enough information to break her and for the grief spiral to kick in, and she isolated herself/was isolated from any possible support.
The character's flawed, yes, but those flaws are more relevant to overall approach than to "would they make a good soldier". It's irrelevant because if either of them sees combat it'll probably be because they're guca at that point and/or we've screwed up unimaginably badly. Normals in magic armor are a bad, bad idea for a lot of reasons, not least of which we really have no reason to do that to either of them.