Personally I always though that purity is imbalanced, even without taking battlesuits into consideration.
1) Best R1 affinity tech, which gives Gene Vault, which is a fairly cheap and relatively good wonder to have if city lacks grassland terrain. Harmony also has nice benefits on R1 tech, and Supremacy R1 tech got zero value besides affinity exp.
2) Fairly easy and reasonable to grab EctoPod from there. Other affinities don't gain that much from rushing Genetic Design.
3) Purity "superfarms" term hadn't came out of nowhere. Fully powered, +3/1/1 (with Ectopod) for a generic tile is absurdly good. Yes, you can praise xenomass, it's really good, but that's a relatively rare tile. Planning your city infrastructure around rare tiles provides unstable results. Basically, a purity farm even without ectopod will almost always support an additional academy, so you're not bound to grasslands and riverdeserts for your academy cities like other affinities. Coincidentally, Cognition is pretty easy to grab to, since it branches out of Genetics.
4) How good everything fits on tech web for purity. Probably the only things that fall out and typically never grabbed of general route is Surrogacy tech and Hyperconductors.
5) Purity has the most R2 affinity techs, which is why purity will probably be slightly ahead in affinity point.
6) I still didn't compare all of the units well (yes, after 120 hrs into the game, I'm a slowpoke), but purity is at the very least not behind every other affinity in that regard.
Seriously, the only real downside for purity is it's 'peaceful' victory condition. It always takes at least 20 turns, while other could be significally shortened (Yes, I'm looking at you, Harmony. Oh, don't hide behind the bushes, Supremacy, I can see your ANGELs from there anyway), and you actually need magrails pre-built if you don't want it take much more than 20 turns.