If my group of five is facing a group of twenty enemies that have the same HP/damage mit, damage, speed, and abilities* as I do, then my best bet of surviving is playing in a very conservative and tactical manner. This slows things down.
If that group of twenty doesn't have the same HP/damage mit, damage, speed, and abilities that I have, then that is asymmetrical.
*Not meaning 'exactly the same powers' but powers that perform the same functions. Example: if I can heal and 95% of my enemies cannot, I don't consider that symmetrical at all.
In which case, the enemies should do 100 damage and I should have 10 HP if you want symmetry.
To survive that, I have to approach the encounter in a very deliberate manner, pausing constantly to heal/buff/maneuver or setting up alpha strikes.
Combat becomes tedious and crawls along.
Your definition of symmetric is literally correct, but it's not what people (well, me at least) are meaning when they're asking for symmetric combats. It's not about the two sides being equivalent, but about them running on the same rules
In a "symmetric" system, maybe a Goblin has 50 hitpoints and does 10 damage per turn, while the PC is a lot more skilled and tough than a goblin, and thus has 200 hitpoints and does 30 damage per turn. So the PC kills the goblin in two turns, losing 10% of their health.
In an "asymmetric" system like DA2 had, maybe the Goblin has 500 hitpoint and does 10 damage per turn, while the PC has 200 hitpoints while doing 300 damage per turn. Which of course has the same outcome - until you bring in friendly fire, because now the player's attack will instantly kill themselves or an equivalent team mate.
DA2's designers seemed to choose to be asymmetrical in order to allow the player to have a sense of progression by letting their damage numbers get huge, while keeping the players hitpoints relatively small, which helps Constitution remain relevant at high levels. But they could achieve the same objectives by having the hitpoint bonus from constitution scale along with level. Or they could keep the player damage levels from scaling so dramatically.
In addition, DA2's hitting mechanics were asymmetrical because the players' attacks delivered damage immediately and thus could not be dodged by moving out of the way, while the enemy attacks did damage at the end of the (often lengthy) animations, often allowing the player to avoid damage entirely with a little attention. Except for enemy archers, which IIRC were undodgeable and thus needed to be killed quick.
Kiting is interesting.
I consider it a valid tactic, but it can quickly become the one tactic you use over and over because even though it's not that fun, you always win with little loss of health/resources. I recall the original BGs where I'd set out with hundreds of arrows and return with none because a couple bad hits could easily kill a companion, and I didn't have any resurrection spells.
At the same time, in a semi-open world game, it's important that the PC be able to disengage. Mike Laidlaw has said a couple of times that you can wander into an encounter far above the PC and they've made sure you can run away if that happens.
Kiting can be a good part of a system if it's challenging, and if it's not a one size fits all solution . But slow, avoidable attacks make it a simple, one size fits all solution.