Yeah, summoning needed some limitations, however the ones introduced with the GH are horrible. The actual main utility of summon, is that in matched play you can bypass limitations on the number of certain units (behemoths)
Hell even you can only summon x% of your starting point total would be better than havig a reserve. Combine that with summoners being fairly expensive (e.g. a make a summoning wizard cost a twice what an equivalent wizard that can't summon costs) and you've got a better method. It'd prevent you from losing a massive reserve to a Lucky assasin, allows you to have a meaningfull amount of summons for any size & it'd still prevent people from just spawning summons at will while the additional cost gives an meaningfull tradeoff.
Summoning is an odd duck. There is a big advantage to not having your army on the table immediately but summoning is not as powerful as say the Stormcast, Skryre or Fyreslayer type abilities (or chamos/shadowstrike). However, unlike with battalions (which cost extra points as a downside) you always need the summoner on the table before you bring things in. This makes you vulnerable to alpha strikes. Also you really need to pay attention to casting values as you won't always have the constellation and astrolith to make those 10s into 8s. There's also an economics advantage. I only own 6 rippers, 4 kroxigors, and 1 bastiladon. But if I can trade some of these pieces at a good rate (1:1 or higher) then they suddenly become available to me again whereas if I didn't have summoning I could never field more than this without buying more.
As per rules of summoning, the summond unit can't move in the Movement phase. But it can charge.. so a summoning of 3 Kroxigor or Rippers 18 inch towards the enemy in the first or second round means a bigger chance of them getting into combat at full strength. Im also tinkering with the idea of summoning two razordons (as I have two). That's 4d6 shots Out of the Blue on an unsuspecting target. and a possible charge afterwards.