I usually use muffler pressure on my Sportsters without any issues other than my .65 runs a couple hundred rpm faster without pressure. Fuel mixture stays more consistent full tank to empty using it; without pressure the needle just needs to be opened up a little more. My .20 runs fine with or without pressure, glow or diesel works the same.
My .65 kicked a prop once - it got a little lean at the bottom of the tank. Sadly I never was able to recover the prop or prop nut.