Actually Ia's not overrate, because her overall quality, vocal and tempo range are better then other Vocaloids around, save a few. She does have a extra vocal for rock, but is fully capable of being used solo without it, the rock is for styles the normal voice can't cover. She does more roles and can fill up 16 tracks of the softwares set up more then happy, which even Miku can't always do depending on the voicebank. ITs not a big deal this particular traits, the first vocaloid to be able to do that was basically miki in V2.
So... If one has to pick on over rated vocaloids, that would mostly the likes of Gumi, Miku and a few others who crept out of V2 and no longer match up to the modern vocaloids... but are still kinda around mostly due to popularity and not much else... IF we're talking overrated vocals, the Kagamines had a crap reputation in Vocaloid2, but their not also considered overrated because everyone knew they were bad, unless your the kind of fan who thinks their the greatest voicebanks ever made.
But back onto general V2's... They don't have the vocal range and are old tech from a time when the average no. of layers was about 2. This is why they have small vocal ranges for a part, plus they tend to have to rely on their extra vocals to carry them further then they can do with their base vocal. If we're talking Gumi, then the original V2 vocal was pretty LQ, as was the original Miku vocal. But this was a by-product of being in the fist half of the Vocaloid2 engines lifespan, things improved in the last half of its lifespan. Bare in mind also its pretty well established in regard to V2 which Japanese vocaloids were the best of their era, with Luka being the worst of the best, and in no order, the rest being the Appends, Iroha and the VYs. But all of those came out in the latter half of Vocaloid.
Vocaloid3 allowed for more variety of vocals then before and a number of newer vocaloids far out did the vocaloids of the V2 era, you've got vocals like Ia, Yukari, and Kokone with vocals that either had big ranges and tempos, or were built for niche genres that they held tightly with a iron grip. All decent vocals within their own right that knock socks off of any single V2 vocal, even when those vocals went V3. A lot of the V2 vocals often turned out to be just reused with triphones added for V3, some bits of cleaning but they were still more or less the same vocal.
The producers who make the songs do as they please, while the vocaloids are just vocals people use for the purpose of song writing. So any vocaloid can be used by anyone willing to buy the software... Or download it illegal but we don't recommend that here for reasons that are obvious. Either way, Ia, Miku and other vocaloids are not responsible for anything that happens to their vocal.
I would just ignore the brats, as often they haven't the foggiest idea what their talking about. IF you asked them which was the higher quality; Miku or Gumi, then they'd prop. say "Gumi"... To cut a long story short... ITs actually Miku despite Gumi being the more realistic, because of the problems Gumi has are far worst then Miku's overall. But there isn't much in it at all...Their both LQ vocals. Their Vocaloid casual fans, not Vocaloid nerd fans... So I just would naturally expect the shallow answer and would be surprise if they can even list what the issues like tone collapse which plagued V2 vocals.
Edit:
I apologise if parts of this aren't well constructed, I wrote this last night past midnight so sleep wa skicking in.