besides being "supports" that goes against the grain AND being supports that got a literal item made for them 
there is also ANOTHER thing about them that gives them a bit of...special treatment.
While most supports have their gold income gutted after their item reaches max upgrade, Senna and Pyke have ways to earn gold afterwards
Curious about Senna because IIRC she doesn't have anything special unless you're talking about gold efficiency from her passive or something like that.
Pyke's whole niche is gold generation, if he doesn't land his ult he goes from a monster to an incredibly lackluster version of Thresh or Nautilus. He's meant to be a high reward champion for those who can consistently land the ult.
Something I noticed about two...problemish supports 
& 
can solo a carry. CC & Damage I'd take a fed Pyke and senna any day.
Every champion has its strenght and weakness. Of course they will have some edge over other supports , other supports has edge in some aspects too.
Senna and Pyke generate their own gold because they are designed to be a hybrid between a support and a class that needs gold to scale.
The alternative is for them to either be adept at that role without needing gold (busted) or a worse version of that role (useless).
By giving them gold in a way that's skill dependent (Pyke needs to land his ult, Senna needs to position well to safely pick up mist), they are given the capacity to scale from the support role without making it guaranteed or requiring them to take kills/farm for themselves.
is just an ult machine right now.
usually get like 200-300 gold from wraiths per game. That's not really relevant.
