Downsides?
(a) They’re not very warm. Size up and wear proper thick socks (they don’t have to be rambler red)
(b) The “cuff” is quite wide compared to, say, a MW80. Basically, it’s easy to get water/mud/sand in the top of the boot. You could try a small gaiter or make a “gasket” from the wrist off a rubber glove (Marigold Yellow – but only if you can ride to deserve it).
As Deveron53 says, any GTX liner will eventually leak. If you can picture that the soles of your feet are repeatedly pushing tiny bits of grit into it, it’s not hard to see why. My MT90s are probably 5 years old now and I still wear them.