I would go so far as too say that assaults in general are the hardest mechs for beginners.
You are slow, so a lot of smaller mechs can just circle around you and chew through that juicy rear arm or. That slowness also means you need to know where you are going because otherwise chances are you won't contribute much because you are at the wrong place at the wrong time. It is harder to shield because your torso moves slowly and you need to be more aware of your surroundings because you can't just turn around in an instant and run away.
Also 20sec is 2 turns in the
While MWO has many flaws I do love that they managed to make light mechs viable. A well piloted light can be a terror on the battlefield and can actually do some of the fluff stuff that the turn based nature of the tabletop doesn't really let them do.
I'd say heavies are the easiest weight class to play by and large. Mediums seem to be in a weird spot, because they mostly lack the speed to avoid taking damage and the durability to survive the beatings.
But then, in a Stormcrow I'd take on almost any mech and have a very decent chance of winning. Unless they have ECM while I have streaks.
And there are a couple more exceptions.