The heavier the bike the heavier the bike rack is going to be to support the weight. Also there are a few bike racks with ramps (Thule, Yakima, 1up) and this makes it a lot easier to get a heavy bike up on the rack. 1up provides the longest ramp which is part of the reason I bought one of their two-bike racks.
A lot more rack options with a e-bike that weighs less than 45 lbs which you will see when you check the specs for the racks.