RT-AC5300 is the worst router ever

OMG, days i have spent messing with this router, and I can come up with only one conclusion that it is not fit for purpose. My 87u works like a treat out of the box with Asus or Merlin firmware. This one with same settings in same location and the wireless is spiking up and down with about half of my speed compared to the 87U. 2.4ghz works, but 5ghz...a disaster no matter what channel and width!
I am no expert, but whatever firmware I flash doesnt make a difference apart from the newest...

RT-AC5300 is the worst router ever