It's quite possible that, in practice, certain phase lengths generate higher or lower rates of NMRs. I don't have stats on that, I don't know if they exist. NMRs are obviously not optimal. It's a person-issue, though. Maybe less people can/would handle 16 hours than 24, but that's not really an issue with the 16 in itself, just an issue with player availability. Not like 12 hour turns, imo, which themselves can be very restrictive on the average person, just due to issues of work/sleep/eat/family/etc.
Now, again, I've only been (back) here for a week, only played a limited number of games, and I can't see what other players are doing (or not), but it doesn't /seem/ to me that everyone that's ready will indeed check "ready". Indeed, someone told me that I "shouldn't have readied up so early in the phase" once, despite that, to me, it was the beginning of the game, and though I had already messaged pretty much everyone, due to the current layout of the map, it didn't really seem to me like anything I would say or be said, on that specific turn, would have any influence whatsoever on my orders. It seems to me that, not so infrequently, some people are prone to not check ready, even though they are, for a number of reasons. Either to stall to have more time on the next phase, or, more likely, as a ruse, to pretend they are more open to negotiation than they actually are, or to pretend that they didn't see the messages they received since the last exchange, or other such tactics. In cases like these, where everyone already put their orders, and everyone's already done talking, then the excess phase time is really just stalling the game's progress.
This isn't really a critic against these people, though, and I wouldn't say I'd never do the same. But overall, and that's just a personal preference, I prefer the "whoops" of legitimately not seeing a message in time to change orders, but having the game be more dynamic and make more steady progress, than having infinite time to work out every nagging little detail of any possible agreement, agreements which one, or both parties likely don't even intend to respect. It's not really that one is better than the other, I'm just stating a personal preference. I'd /rather/ play 16h, but since 24h is the norm, I don't really have a problem with joining 24h games.