I think you're underrating infantry, because you don't build them for their best trick: Suppression + Lock&Load. Once Ammo Conservation gets invented, such an infantry can sit in heavy cover and slowly solo kill a Cyberdisk or (unmerged) Mechtoid with almost no risk, by alternating (Shoot->Suppress) and (Reload->Suppress). When the alien forces start to rely more on "bosses", the ability to have a single soldier who can lock down a single enemy forever becomes very very powerful.
I think you're underrating infantry, because you don't build them for their best trick: Suppression + Lock&Load. Once Ammo Conservation gets invented, such an infantry can sit in heavy cover and slowly solo kill a Cyberdisk or (unmerged) Mechtoid with almost no risk, by alternating (Shoot->Suppress) and (Reload->Suppress). When the alien forces start to rely more on "bosses", the ability to have a single soldier who can lock down a single enemy forever becomes very very powerful.