Viper feels really bad below 90, and basically unplayable below 80. Reaper is a bit better, likely because it was made when the level cap was lower. It feels pretty ok once you get Gibbet and Gallows at 70, and is pretty much feature complete by 80 after getting Enshroud. There are some other classes (like Ninja imo) that genuinely feel pretty fine even at 50.

I do think some of this has to do with how Viper is just a weirdly (some would say badly) designed job from the get-go. It has so few buttons as it is that once you start to take those away you really start to notice the cracks in the design. On top of literally 50% of it's rotation being on just two of those buttons it really creates a problem unique to Viper. It's not really a problem you can easily solve without either overhauling the job, or waiting a few expansions for it to get more buttons then spreading them out over the levels a little better. But at that point the level cap will be higher so we may just be back at square-one. Not enough buttons to spread out to all levels of play.