The real thing everyone should know is that balancing is HARD. As someone working towards entering the industry, I've had to do some practice for balancing, and it is much harder than you would think.
I'm not sure anyone is saying game balance is easy, but I should only really speak for myself, and my problem is these are all valid criticisms that I echoed through alpha, beta and more recently, in another thread. To see the game released as-is, following a relatively balanced Divinity: Original Sin Enhanced Edition and leaving much of the same feedback beforehand is frustrating admittedly.
The problem is two-fold.
The first problem is the new armor system (which is actually where most of the balance issues throughout the entire game probably come from, but that's beyond the scope of this post and is probably beyond fixing at this point, short of some massive game overhaul; Divinity Original Sin 2 Enhanced Edition, in other words, but I digress).
Anyway, the new armor system significantly diminishes the value of crowd control and lowers your ability to maintain control over the battlefield. The easiest way to counter this then is to simply overpower enemies, and the best way to overpower enemies is to increase your available action points usable toward overall damage per round, so you can nuke individual enemies by focus firing them.
Now if we were to stop right here and call it a day, some people might think that's okay. Personally, I dislike this type of system greatly and much preferred how combat worked in the first game, but that's me.
However, your ability to maintain control isn't only dependent on your capability to deal damage and quickly remove enemy units from the battle. You also need to control your enemies' ability to deal damage to you, so you don't get blown up, since recovering is also much harder than in the last game.
So what is the best way to increase your overall damage output and mitigate overall incoming damage without CC? Summoning. This is the most effective way, by far, to increase your available action points usable toward damage and increase your overall survivability on the battlefield. By increasing the amount of attackable units on the field, you increase your overall health pool, lower your chances of receiving multiple negative status effects at once on any one unit, and increase your overall flexibility to deal with RNG.
For these reasons alone, Summoning is the best skill, but then throw in the scaling it provides per point, and it is game-breaking. Game-breaking as-in Ifan's Source wolf Ifrit can almost deal as much damage with its Bite attack for two action points as Sebille can with Flesh Sacrifice (which provides a damage boost) followed with Backlash (guaranteed critical hit), Throwing Knife (critical hit), Adrenaline Rush, Sawtooth Blade, and a normal backstab attack (critical hit) combined for a total cost of seven action points.
I realize this comparison seems "unfair" or some may argue it "gets better" later on when the massive stat inflation from gear kicks in, except that it doesn't. Ifan's wolf (or any summon, for that matter) is bonus damage. A truly fair comparison would be Sebille later on with massive stat inflation -versus- Ifan's wolf -plus- Ifan with massive stat inflation -combined-.