I don't know why you're comparing Salamence and Hydreigon when they both do two different things. Hydreigon's bulk is what separates it from other Dragons, as well as an abnormally large movepool.
In terms of dealing with Genesect, they both have their own merit. +Speed Nature Scarf Salamence will always outspeed Scarf Genesect, but you have to be wary of spamming Outrage, as Genesect can switch in after you've killed something and easily revenge you. Hydreigon on the other hand has enough bulk to switch in and take multiple Thunderbolts / Flamethrowers, as well as being able to Roost off the damage. Should Genesect switch into Hydreigon, it has to fear Hydreigon setting up a Substitute and then Hydreigon being able to KO with a Fire move the next turn.
Keeping on topic, I think the one thing that really lets Genesect down is that base 99 Speed. Not being able to outspeed Scarf base 100s such as Jirachi, Victini and Salamence in particular is pretty annoying, especially when all of them can dispose of Genesect with the correct move.