I'm currently running mage and summoner on Expert mode with a buddy and I have a couple of concerns in terms of balance, progression and item acquision:
1. First off, I had incredible problems getting a lot of stuff as a summoner. One doesn't simply get started as a summoner, almost all summon staves are monster drops, the only pre-hardmode armor is requires a queen bee kill. It would be really awesome if slime staff, spider staff and some other basic-tier summoning items would be craftable, I have killed over 150 enchanted swords and still lack the dagger summoning staff, twelve dreadnautilus and nothing. At least having one staff per tier that can be crafted would be a huge boon, dreadnautilus and enchanted swords are very rare, and the spider staff is the worst summoning item I could think of, which brings me to...
2. The whips are a nice idea, but summons generally lack the speed to keep up with expert mode boss moves, especially melee-focussed summons like spiders can't hit slime queen, skeletron prime or the destroyer effectively. You'll also hit many enemies at once and really getting your summons to focus would require you to keep the whip equipped and only hit that one segment of that one enemy without attacking ads or different segments. It feels a lot more effective simply using a ranged or melee weapon while having your summons deal some damage. This can drastically change when the stardust cell and dragon become availible, but pre-stardust-level I generally feel a lot less powerful than a ranged or melee character. As soon as I had a dart rifle and a frost armor any expert-mode mechanical boss and slime queen would melt in a minute or two while my summoner (Forbidden set) dealt only about 25% of his actual damage with the summons. One thing that adds to this is the lacking availibility of summoning acessoires or items that boost summon reaction and movespeed. It's neat to have you back covered while fishing, but when the bosses come out and my performance really counts I still felt slightly underpowered, even with the summon tick adjustments. Point one comes into play a lot here, having good equipment at your disposal should not be a question of RNG only.
Edit: I forgot one litlle oversight: The Lifedrain spellwand still doesn't drain life and is underwhelming considering the mana cost. Can we please make it actually convert damage it deals into life? On expert the life regen is not even noticeable
