So far I think that the "over-rated" term is being used as a forced synonym of "the games made by that dev are really bad", rather than simply being "they're indeed good games, but perhaps they're being given a bit too much credit or praise, but they're still enjoyable". That or maybe it's just too subjective and I just happen to have my own definition of what is a developer (or just one game) that happens to be over-rated. And I guess that the same goes for under-rated.
I mean I pretty much disliked Mass Effect 3 almost completely (save for the Tuchanka arc and... that's about it I think, being completely honest here) and the "choices" (I.E. lack thereof) given to our Shepard(s) from the bony dialog wheel were shallow or sometimes irrelevant to say the least, but I wouldn't say that BioWare as a developer is "over-rated". They still make superb games overall, usually still better than most and the production quality is still evident, even if some specific games or moment in those games end up "more cheesy than BioWare would normally do".
That's the problem I have with "over-rated" and "under-rated", lots of people use these terms to just bash, rather than give credit where due but still point out at issues that maybe some people didn't consider when originally giving scores, high merit, or themselves being hyped without looking at the details during previews or interviews, etc.
Anyway...
For me as far as over-rated goes I'd say Blizzard, since I really think that Diablo III was evidently released too soon, barely completed (if that) and with obvious lack of preparation on Blizzard's side during release (Error 37, anyone?). They had... oh, what... nearly a decade to prepare? When they said "we just couldn't expect that much demand" (in relation to server issues, latency, errors, etc) I couldn't believe it myself. I mean it could have been a genuine naivety, you know they might have thought (honestly so) "well, ok guys we're ready, that's for sure!" but c'mon, it's Diablo III, the numberless hordes of Diablo fans all around the world waited for that moment for a decade. Sure no one knew in advance before playing it that it would turn out like a failed attempt at even being a Diablo game (in my opinion, anyway, and remember I'm referring to the build we had at release, now the one we have now) but prior to release it was a genuine hype almost without precedent in the gaming industry that didn't just start to build up a week prior to release, it built up before D3 was even rumored to be in development. To be honest it just felt like an uncared for and unprofessionally barely-prepared launch of a clearly rushed, uncompleted game.
Now of course today D3 is "better" than it was at release and I DO enjoy some of it, although with very controlled moderation because, honestly, I DO get bored of it if I just dare playing more than two hours. So when I feel like just burning some time on a rainy day by bashing some skeletons in a crypt I just launch it get on my Barb and clear early Act 1 for maybe an hour and I get my fill. I do NOT "hate" the game, nor do I "hate" Blizzard, just want to make that clear.
And under-rated, for that I have to go back a couple of years to Arkane Studios' Dark Messiah of Might and Magic. Mostly due to that game that is, so of course I could as well say that what's under-rated is that game, rather than the developer themselves, because we know that recently enough Dishonored did quite well in the market. But had it not been for Dishonored, honestly, who would have remembered about Arkane Studios? Well I sure do since I really liked Dark Messiah at the time, and still today I do. When a thread asks for sleeper hits or under-rated games I always mention Dark Messiah, it's a reflex by now, barely ever mentioned, easily forgotten and in my opinion unfortunately disliked by too many whom probably never got to the end of it to start with, but that's ok to each their own.