I'm not an expert in this area at all but I recall reading that trying to hide a signal in random noise doesn't really work, as you can still find the signal with statistical analysis or looking for distingishing characteristics that are not obscured. That was my first thought when I read about this new feature in OpenSSH, and it seems to have proven correct.

Edit: wanted to add that I recognize that the people working on OpenSSH know a lot more about this than I do, and I had assumed they wouldn't bother implementing this if it wasn't a good idea, so willing to accept that "proven correct" may be an overstatement or even flat-out wrong.