With the release of the new Set, there's bound to be issues, everyone knows this, that's why it comes to the PBE so these issues can be fleshed out and balanced. The biggest issue I've seen thus far as far as comps and builds go is 100% Summoners. Why?
Their damage is incredibly over scaled for Champions who are summoning smaller, weaker units. A 2-star Zyra can wipe an entire team with her plants. With the loss of buffs like Knight and Dragon buffs, this is too strong. It forces you to rely on Dragon Claw or praying to god you can build a Hush on every unit. Annie is by far the worst, with Tibbers having a massive health bar and doing 500 damage per hit on top of spawn in damage.
The amount of units spawned is ridiculous and the frequency of their spawn, Malzahar alone can continue to spawn voidlings given he has time to accrue the required mana. Before you know it, the board is covered in voidlings and the corpses of your dead team.
Consensus: Champions whose main AO being summoning small, slightly beneficial units, should not be able to bring in units that can wipe out an entire team. These summoned units should be doing a fraction of their current damage, it is way too high when 2 0-star voidlings can kill a lvl 2 Ornn in a matter of seconds. A 2-Star Annie can call in a tactical nuke with Tibbers, he spawns in, does splash damage to those around him, and will attack and deal 500 damage per hit, that's 3 hits on one champion with 1500 health to kill them. If you were to play Summoners/Ocean/Mages with Mage's Cap on Annie/Zyra/Malzahar, you are guaranteed a win. This class is far too strong and needs to be dialed way back.