Out the box Asus's do not support MER / Option 61 so yeah, you need to flash them with Merlin or similar.
My RT-AC68u which is getting on a bit now sits on 5Ghz CH100 and I generally get around 600Mbps over wifi upstairs. Pretty good really.
As Sky do not offer anything faster than 150 at the moment, i'll keep the RT-AC68u for a bit longer. Should they offer 900Mbps then i'll go with the Asus RT-AX86.
I have been using Asus routers for years and have personally found them to be incredibly stable and far better than the stuff the ISP's give you.
SkyUltrafast FTTP 150