Tissue Engineering (Harmony): Units heal +10 HP per turn
Swarm Robotics (Harmony): unlocks Drone Sphere
Metamaterials (Harmony): unlocks Tectonic Anvil
Alien Biology (Harmony): Allows Workers to clear Miasma. Workers immune to Miasma
Alien Adaptation (Harmony): unlocks Xenoswarm unit (requires Xenomass, can be traded or from Affinity benefits)
Alien Hybridization (Harmony): Units immune to Miasma; Units heal in Miasma
Designer Lifeforms (Harmony): unlocks Rocktopus (requires Xenomass, can be traded or from Affinity benefits)
Photosystems (Harmony): Unlocks Solar Satellites, improves Plantation output
Swarm Intelligence (Harmony): Unlocks Bytegeist
Nanorobotics (Harmony): +1 Science to Manufactories
Exotic Matter (Harmony): Unlocks Ansible
These constitute most of the techs that award Affinity points. Many do not require having a Xenomass tile to be beneficial. Alien Adaptation is a leaf tech of Alien Sciences, which does require a Xenomass tile to make sense in itself. That's about it. Both Xenonursery and Xenofuel Plant are from Alien Sciences - the quest that boosts Xenomass tile output is from Xenofuel Plant.
I don't actively avoid taking Xenomass tiles when I'm playing Harmony. In fact, I seek them out! That's because the techs I have make them very, very good for me. However, that does not mean that my faction plays exactly like a Purity one just because I don't start with a Xenomass tile. I hope I've demonstrated that.
So basically I don't really see the optional miasma bonus perk as something that is inherently tied with Unique Units, and I see very little reason not to want to get Xenoswarm since they are still stronger than anything else you can get at the same affinity level. Besides nothing says "harmony" better than a unit composed of a mix of aliens and humans.
Likewise, I don't actively avoid building Xenoswarms when I have the tech and the excess Xenomass. It's just that, for the most part, they kind of behave almost exactly like a tweaked Battlesuit. A Battlesuit is at least distinguished by being the most powerful Strength Aff4 unit, capable of withstanding a great deal of abuse, and dishing out lots in revenge. A Xenoswarm is kind of just a disappointing version of that, graphics aside. I mean, you can easily overlay that graphic on anything.
It doesn't feel very "Harmony." The XenoCavalry does because when upgraded, it ignores terrain penalties. It can hie off into thick jungle and move very fast. I'd have liked something of similar distinguishing nature on the Xenoswarm. As it is, it's just a Miasma-loving Soldier with fancy graphics. It doesn't even inherently heal in Miasma.