Trundle sup = Good, Sion sup = Troll, ???

Not saying I have any personal issues if you want to go Sion support, but there are some pretty clear reasons why Trundle as a support can be viewed as far superior in his current state.

You may as well engrave these words into your mind when you approach any discussion about what are perceived as more viable champions in a specific role:

Reliability and Safety

Trundle in the support role has these in boatloads -Decent base stats -Decent early game trading in lane -Regeneration which is not reliant on his own attacking/farm -Ease and safety of using his E pillar to split teams and disengage. -Ultimate is not very reliant on his own stat growth to be useful.

Sion, on the other hand -Must remain close in the fray to apply utility -Poorer base stats -Less reliable damage trading -Less reliable CC

"Scientists at the University of Leeds have conducted research that proves the tendency many have to act like sheep, unwittingly following crowd as if they didn't possess a reasoning mind"

Sion support is fine, used to play it all the time. I honestly think he's a great candidate for support because he doesn't need top lane farm in order to do what he does best. Everything he's good at can be accomplished from the role of support.

Trundle has a pretty good disengage and will make the enemy ADC pretty much useless since he steals an enemy target's AD.

Sion can only poke and disengage at best.

Though I have been ran over by a Sion support once, his shout "poke" chunks down your HP pretty fast.

I wouldn't call Sion support troll but he is much more farm reliant than Trundle. Sion's base stats are very weak unless he can gain health using his W passive from killing minions.