It seems like it's not used at high level. Is it still a useful pass? Seems to me it's harder to hold someone in place long enough to pass successfully.
I use it very successfully. I'm a 4 stripe blue belt. I have a high success rate at my level, I can occasionally use it to pass purple belts and have part a few brown belts with it. I have the hardest time with powerful people fully committed to the framing me to prevent me from getting upper body control. Sometimes I can just mount the legs and climb up from there.
Yes it is a useful pass for no-gi. I personally don't do a lot of over-under passing myself, I think it takes a strong neck, and also like any other pass you need to invest a LOT of time to perfect it if you want to use it against good guard players. But when I play guard, it is one of the more difficult passes for me to deal with because not many of my training partners do it well in no-gi.
As with many other passes, once the top player gets to a certain point, it can be very hard to deal with. In the case of over/under pass, it's when the top player's shoulder settles on my stomach. So for me the key is to block the initial entry so they don't get that nice shoulder-on-stomach position.
One of my partners has a decent over-under pass. I don't train with him often, so I often forget to frame to prevent his lightning-fast "dive-bomb" entry into his over-under pass when I spar with him for the first time after a spell. Once I spar with him a few times, I don't have any problems with it because I start to recognise when he's going to attempt to dive in, and I set my frames a lot sooner. That buys me an extra second to set up all of those classic counters vs over-under (such as: the "shoulder drag" which is my favourite, the Roleta / backroll sweep, the "flower sweep" with bicep control - I don't know what else to call it, the shoulder crunch or kimura, etc).
For a lot of other passes -- stack passing, leg drag passing, knee cut passing, toreando passing, half guard passing, etc -- I see them basically every day, many times, and I have "automatic" reactions such that my frames can consistently shut down the initial entry without putting too much mental or physical stress on me. Especially vs toreando passing, which is one of the most common pass attempts out there, I barely have to think about setting frames, even when I'm actively hunting for my own attacks. But I don't see over-under passing much in no-gi, and I just don't have the sharpest reaction against a fast dive-bomb entry (after not seeing it for a while).
I think it works no gi, but IMO it isn't as powerful as the gi version. I use the no gi version, but I feel I need to be a bit faster and more willing to let go of the position in no gi because the grips will easily slip off if everything isn't perfect. I think it still worth putting time into, but I personally wouldn't make it my #1 no gi pass.
I don't know about it not being used at a high level. Bernardo Faria won five World titles and six Pans golds using it as his primary pass. It has also been used a great deal by other high level competitors such as Lucas Leite and D.J. Jackson.
I think that it is more effective in Gi than No Gi, but that is not to say it does not work no gi. It is, however, a very technical pass. If you are missing any of several key details, you will struggle to make it work.
lastly, let me second diiddyy's thought: if you have not seen Bernardo Farias instructional on the subject, take a look. He was the best to ever do this pass at a high level.