I also believe than a mixed damage group is WAY better than a pure physical one (and than an pure elemental, of course).
It's not. My first group was (Two-Handed after a restart, when realising how bad sword and shield is in this game) Warrior, Ranger, Pyro/Geo/65% crit/Scoundrel caster with a Summoner/Geo/Hydro/Poly group. They struggled during the earliest levels, but then as soon as Ifran's BFG was unlocked, it got drastically easier. Other than Fireball and Searing Daggers, caster damage is pretty bad until Act 2 in general, even with combinations. The key to this team even working was the Summoner too since it could either have a physical Incarnate, or an elemental one. To add insult to injury, the Incarnate will do more damage with Fireball and Epidemic of Fire than what an elemental caster, that's focused purely on Pyromancy can do at the same level for a long while. Epidemic also costs less to cast this way than if the character tried casting it...
Slow and Cripple were useful against melee, but that's about the extent of manipulation that mattered. Teleport has too long of a cooldown to be reliable, and Netherswap never seemed to work at all the way I needed it to.
1) They can control terrain way better than physical character.
And this doesn't matter in the slightest. A dead enemy doesn't reach you anyway.
2) They have WAY more powerful AoE damage than physical, and sometimes very easy to setup too (Hello Aero ?)
Arrow Storm says they don't. Barrage and Ricochet are also comparable to Fireball. Thunderstorm, Hail Storm and Meteor Shower are the most damaging AoE spells Mages get, and they are AP/Source hogs. Plus, does it even matter? Most encounters have enemies that are spread out and they come in pairs. They only group up to take advantage of much of anything other than Thunderstorm when fights drag on aside from a few, very specific encounters.
3) They have elemental affinity which IS a big factor, I wonder why you're dismissing this so easily ? It makes for insane burst potential and/or utility. I dont think physical have that kind of option.
Insane "burst damage" that requires spending extra AP to setup in the first place. It's not efficient until the fights start dragging out. Elemental Arrows (
especially on an Elf) will actually add more burst damage at the start for 1 AP.
4) Synergy between physical & elemental build are powerful. Some targets are way easier to deal with in a certain way (pure physical, elemental, one specific element etc), and if you have a mixed group, you can take advantage of this.
Nope, nope, nope. My second playthrough was 2x Rangers, 1x Rogue, 1x Summoner/Ranger (who would have been better off as a more focused Ranger most of the time. Biggest issue is finding three near equivalent bows every few levels) and they blew through high physical/low magic armor faster than what my caster was able to do to low magic all throughout the entire game. Split damage can work fine, but it's still
much worse than an all physical group, even against higher physical armor.
Your target is more sensible to elemental and your warrior looks helpless ? Not really, use them skills to enable yours mages (Phoenix Dive on your Pyro, Flay skin / Medusa Head / Oily blob).
Or simply kill them instead.
Your target has a lot of resist / magic armor and your mage feels sad ? Enable your warrior / rogue ! Haste, PoM, Firebrand, Encourage, Teleport target at the feets of your Knight, block terrain to isolate the target, slow/cripple other targets etc. You can even use Boucing Shield for a more straightforward physical support.
Or simply kill them instead.
I started a 4x elemental caster group recently, and they were an absolute nightmare to play compared to the previous two. Easily the least fun I've had with the game, and the only reason I kept going was hoping that there would eventually be some kind of payoff. There wasn't. The mixed group worked better, but that was mostly because of the Ranger and Summoner, while the second, all physical group was a night and day difference, and clearly better than any other combination (not necessarily the builds, but simply being physical).