I have a strong Dark Side since I invested in DS toons way more than in LS, the first 5 phases are quite easy for me. In phase 6 however I cannot even kill a single enemy in wave 4. "Classical" First Order (no KRU etc.) with all Zetas and 3 of them G12, 2 G11 with decent mods as well as Empire with 2 Zetas, 3 G12, 1 G11, 1 G10 can't pull off a win there. Sith only managed 2/4 with 2 Zetas, 1 G12, 3 G11, 1 G10.
On Lightside TB I only need a Zeta on Jyn's Leadership ability and 2 G11, 1 G10, 2 G9 to get an easy win.
And in my opinion this is the biggest problem, there is no sense of accomplishment when you are just getting demolished with way better toons, than those you need for LSTB where you can win easier with less effort.
The battles are much harder for DS although they are the weaker faction in most people's eyes as well as being the smaller one. Those rebels kits are tuned strongly against DS toons who rely heavilly on debuffs, but when these get cleansed and the enemies are being healed every 2 turns, you don't stand a chance.
And not being able to fill all platoons because of all those P2W characters makes it even worse, because on top of their strong kits the rebels also get massive advantages through their platoon abilities.
For me Dark Side Territory Battles are fun until Phase 5, but Phase 6 is just frustrating. Because right now there is no way for me to become considerably better, because there is not much room left for improvement when most things are already on their peak, except for the Premium platoon units.
The Dark Side faction as a whole and this TB need an overhaul to be somewhat balanced.