Currently have a race king on the back and mountain king on the front, both ust. Looking for some more grip so was looking at putting a rubber queen on the front and moving the MK to the rear, but its a heavy old lump at 1100g in ust mode, the non ust is 850g what would be the disadvantage of running the non ust tyre, other than a bit of messing around sealing it.

Thinner sidewalls and carcass may make it more prone to snakebites.

Bead interlock won't be quite as good, especially at lower pressures.

In reality you'll be fine.

I run a UST on the rear and a std tyre on the front, both tubeless.

Tried the std on the rear but still got punctures so went for the thicker, more robust UST casing. On the front, it gets less hammer so is fine. I also like the suppleness on the front, for feel and grip.

This is on a 456 with Fat Albert 2.35 front and Albert 2.25 UST rear.