I would advise withdrawing all of your coins stored on Nitro ASAP. There is a fork coming tomorrow. It's a segwit2x fork to make transactions faster and cheaper and will probably be pretty popular.
Over on Bitcointalk, I was finally able to get an answer as to whether or not they sequester their clients' funds 100% in BTC. They do not. They are probably bust. If they converted any significant portion of BTC deposits to fiat, they cannot be solvent. The math is simple.
I expect that the next time there is a major fork, they will be hit by a wave of withdrawal requests and it will all come crashing down. This fork tomorrow could be considered a serious one but was not at all well publicized so I would expect them to be able to keep the charade going for now.
I can tell you from bitter experience that they will delay a withdrawal request for about 24 hours if a fork snapshot is coming. I would not wait if you want your segwit2x btc coins.