It is possible, but it's far from simple. EotG's range is limited, but the model is not small, the enemy needs to have it totally surrounded, if they melee the EotG on the front it can summon on the rearside. With a Slann it's even more difficult, given the 15" range. Once I succeeded in doing it, facing a seraphons' force, but only because my opponent placed the slann in a corner, protecting it with the thunderquake starhost and so i was able to surround the block. With an arcane vassal it would be even more difficult. And against other armies it would be simply impossible (beastmen shaman can summon everywhere in the battlefield, with no range)
Meh that seems annoying, though it'l heavily depend on how easy it is to pull off. I'd rather have seen a limit like "within x" you can summon whereever you want, outside of x" it needs to be at least y" away from enemies". That way you can only dump something on a high priority target by getting your summoner close to it (and probably in danger). Ah well, some of these rules look quite weird on paper, aybe they make more sense in actual play.
What i find annoying is that the hero phase comes before movement. You cannot move your wizard and then cast, so it's always more easier for anyone to keep under control enemy's magic.