I can't speak for others, but I find them annoying because they don't fully dim. There are new versions of those windows that are supposed to get an order of magnitude darker, but I'm not aware that they are being used on UA 787s. The other thing is that they can be under flight attendant control. That's frequently a good thing (dimming windows that sleeping passengers forgot to so that sunrise doesn't blast the whole cabin), but sometimes the FAs just lock them in the dimmed state, which is annoying if you actually want to look outside.