With regards to the 5Ghz, after the recent driver update from Quantenna it's improved a lot. I admit that in the beginning I rather just turned the 5Ghz off it was so bad, however I can't say that I've really had a problem with it recently since the new Quantenna drivers were released in the recent firmwares.
Also from what I read, the current beta firmwares improve this even further.
Can't say I've had problems with the signal dropping though, I'm often at home connected over wifi with VPN and it's perfectly stable, even on the 5Ghz.
It's by far the best and fastest router I've owned. Especially now that the 5Ghz has been improved recently, and still improving, I'm really happy with it.
I've also used the DSL-AC68U (which is just the DSL version of the RT-AC68U) which also works great! It's not quite as fast as the 87U, but I don't think you'd really notice the difference in your every day life. It also uses a different 5Ghz chip if that's your concern.