[Snake]

Just something I noticed. How are even summoners weak?

The pain comes after a Summon Storm, where 7 'glass cannons' (read: paper-worth in HP but still, high damage dealers, often on a minimum of 120 magic damage, aside their utilities like blinding, immobilize or inflicting silence.) come to fight you at once and gangbang you down to death. The sheer numbers plus the fact they may resist one proc of your AoE damage can be annoying as heck. Especially if the Summoner is being a Priest for the Mass Grafts and buffing their defenses with Sanctuary, or giving them a 7 round 'Shukuchi' on Quickness, making it impossible to run away from them.

Now imagine that, but with each Youkai being a player character-worth of survivability and damage? Ew. Just making anything slightly remind me of PvE and grinding makes me sick. Why bring this hell we've been through to PvP? Nein.

I'd only agree on a Youkai being 'strong and tanky' as a player character if there was a Bonder feature at only 1 bonded Youkai, tho'. Maybe a small tweak in Synchro Summon?
Just remove the 'you also gain the effects of this skill' part, and put 'the skill's duration and bonuses are doubled (tripled if 100% friendship) if you only have one bonded Youkai'. And bam, solved the 'Youkai are useless' part of this deal.

Bonders are pretty 'mandatory' to bring out a true Youkai's potential.
G.Summoners should use them like units, the 'grand' part comes from the number of Youkai they can summon at once to overpower the enemy with sheer numbers.