So basically you're breaking it down into two things.. why people dislike Sasuke and if Sasuke is being underrated power-wise.
The former is more subjective based than objective, so really there's no fool proof plan to bring Sasuke onto their good sides. I like discussions where everybody gets to state their opinions and why they think that way, so that's all fine and dandy. However what you are referring to are the accounts that seem to hold some sort of grudge against him, which is a different matter. Well there are possible reasons for this..
> They really do hate his character and are overly expressive about it
> They do it to piss people off (troll)
> A reaction to somebody overly praising Sasuke and flaming other people/characters
It's hard to tell them apart sometimes >_>
As far as power goes.. I'm pretty sure it's the fact that they keep comparing him to his known rival - Naruto. Ever since he got his power-up with the Kyuubi, he's gotten a nice battle record. His clones are capable of fighting with Kage-level opponents and Naruto himself is fighting off Tobi with some assistance (he was the key player for a while). The Kabuto vs. Uchiha Brothers fight however made Sasuke look a bit dependent on Itachi, and constantly made him be portrayed below his brother who he should have surpassed by now. I know Sasuke is capable of more (the conditions of the fight gave him a power limit), but for now he's being painted into the readers' minds as weak.
Of course the haters are going to try downplaying him no matter what. If you want an actual representation of the Base's opinion, only look at the reasonable posters.