Pros
-Low Cooldown CC
-Very tanky with few items
-Ult greatly reduces enemy tanks survivability while making you tougher
-Great early game sustain due to passive

Cons
-Your a tank and nothing else
-No pulling CC
-E if used incorrectly can actually get teammates killed.

Your E is what makes Trundle support as its a massive slow, long range, and low cool down when maxed out. Use this to block pathways, break channels and my personal favorite pop it when a enemy is trying to recall. It also nets you an assist when a enemy killed so spam it without fear of KS'ing