Summoning is kind of weird and feels wrong, but I think it was an effort to try to prevent summoning being used as an insane force-multiplier in what's meant to be a balanced match. I'm not really a fan, but I won't complain too much, because I understand the intent here. As long as they don't up-charge the summoning models, that's fine. Sure, let them pay for being able to bring a unit out of reserves and plop it somewhere (and make sure Sigmarines do the same with units that have those abilities), but don't charge for them like they're able to add to the army.

I think it'll be fine in time. For "fun play," people can opt to change that rule. For actual matched play, it makes sense.

And if people think it doesn't, well, point them to Chaos Daemon armies in 40K tourneys that spam new units.