I am confused at what the heck Niou is doing. Is Niou actually doing these techniques or what? Because I thought they made it pretty clear during the Nationals the he is only doing "Illusions" and not really the techniques. It just looks like he is. That's why Niou isn't as good as the original people he imitates. Though at other times it seems like Niou is really doing the techniques.
Illusion itself is only an optical illusion. Niou keeps all of his stats though and whatever technique he performs, he actually uses.
He can completely copy other players' techniques. It can be said that all of his skills are at a high level, to the extent that he can use 'Tezuka Zone'.
He can completely reproduce other players' moves. His optical illusions have the power to replicate those players, to the point that you think they're standing on the court.
http://fanbook.livejournal.com/56969.html
While that's pretty clear cut most of the time, the auras are a bit questionable. In PoT, you can see auras, so it should also be possible to make them appear with just the optical illusion, and with Saiki for example we don't actually see the effects and with Hyakuren, we do see effects but he doesn't use it throughout the whole match.
Given what Chitose said when he opened Saiki, it might be that Niou can use the doors but not over a longer period of time.
That being said, I still don't think he used a true Synchro, simply because in order to use Synchro both partners have to trust each other. Now you could argue that because his form changes, Oishi and the twins are fooled and trust him enough, but the problem is the other way around.. Niou isn't exactly known as a trusting person. So I guess it's possible that he used a one sided Synchro in both cases, that allows him to influence the other person but he isn't influenced himself.
Which doesn't make sense, because then he would technically be as skilled as the person he is imitating.
Well, look at it this way. Niou can use Tezuka Phantom perfectly during the nationals, but as far as we know he only manages to pull it off for one point, unlike Tezuka who hit it at least 19 times iirc (that plus however often he had to use it when Sanada countered with Rin). If you accept the theory that he can use Hyakuren and Saiki, but only for a limited time, it's again different from what Tezuka can do.
Basically he can use techniques as long as his own limitations allow him to.
That being said, he also keeps any advantages his body has over the person he copies. For example, I wouldn't be sure whether Tezuka would have been able to return Kagero Zutsumi like Niou did.
As a side note, Hitouji's voice imitation is different from Niou's Illusion in that his appearance doesn't seem to change.