[Miryn]

When you say PvE you probably mean grinding. It's normal that summoner classes play more slowly, making them worse for quick grind, but summoner is definitely very powerful in event pve where the goal isn't to quickly clear mobs but to win. The class is slow, but solid. By that logic, healers and support characters are not very useful in grinding but are very strong in events. Also the dark rematches are also hard content that lets summoners really shine because it's all about being able to sustain for a long time, so you can keep sending out bodies to fight and using reihou. It's definitely good in PvE, just slow. At least GS auto summons a guy. Ruler also suffers from this but at least the pawns are not bad for grinding

I mean in higher content in general, The class lacks damage, The youkai can rarely hit anything, Die quickly, and only two youkai become even decently usable. Without Chain unsummon they'd be unable to do most high end content. Add in the fact that Youkai aren't like the other summonable units and have to be brought to the inn or crystal to heal fully between combat makes it very hard to keep doing stuff. A full dungeon is very painful without the power of Chain Unsummon.