I will just summarize this as oppose to writing a lengthy list of abilities and examples…
I think the biggest problem with the siphon abilities is the amount of siphon abilities Necromancers possess. Each individual ability has to be weak, because Necromancers are capable of stack several of them together. Of course, in reality stacking them all together requires a large allocation of resources composed of traits, gear, and utilities that shoots the opportunity cost of fully utilizing siphons through the roof.
80 Necro / 80 Ranger / 80 Thief / 80 Guardian