Installing such app almost certainly implies that the user is aware of the problem of vertical videos. So being aware of it, they will simply rotate the device when recording (since it provides better quality/resolution).
It seems to me that it would be helpful if such idea is applied on the system level without the need for an app.
I'm on the opposite side, several times I tried apps that give only 3-4 frames per exercise and sometimes (if they are not shoot with an appropriate angle) they are not enough to make clear what you have to do.
I don't think they are unfinished, just a new art direction. But it seems to have less neutral style that before and that will reduce adaptability to its surroundings.
Yeap, both setTimeout and setInterval are being sacrificed for performance. Alternatively you could use WebWorkers to offload the timers for precision.