I've already said this elsewhere, but since this seems like a proper place to bring it up:
The one-summon limit is completely awful and guts the iconic gameplay for many sorts of D&D summoners. Additionally, it results in incredibly unintuitive interactions where spells clash with each other due to being implemented as summons, especially Mage Hand and Find Familiar. You can't use Mage Hand and have a familiar at the same time? You can't summon anything else if you have a familiar? Pure frustration.
The entire fun of playing a summoner comes from amassing a swarm and from choosing your summons to have them complement each other. What's the point of restricting them like this when the source material was plainly balanced around having multiple summons at once?
EDIT: Another observation after playing a bit more:
Enemies have generally been given more HP to account for additional ways of damaging them, especially via fields.
However, since spells that lack field interactions weren't buffed to compensate, this means that any spell that does damage or cares about HP but which doesn't create a field is currently very weak. This is most obvious with Sleep, which will almost never be able to hit more than one monster, and very rapidly becomes useless even at that.
