The Androns and the "Medium Drones" were introduced to me in a horrible way during a "city assault" mission, and as I said before, I could barely handle them. I knew that if I were introduced to the next "tier" of alien, I'd be unable to handle it.
The very next mission, I unfortunately was, in an "Alien Base" attack, and they made the Androns look like child's play. I quickly lost my entire squad and thus the game.
It was mostly an economic loss though because even though I was familiar/decent at the tactical/battle portion of the game, I had no idea how to do the "geomap" part. I built my bases too far apart, I wasn't sure how to maintain an economy, I hired way too many scientists and engineers far too quickly, and then I couldn't continue to build bases, afford the new technology, and etc. etc. I fell into an economic spiral, then the game introduced Androns and the next tier of stuff enemies back to back.
I honestly don't know how the game decides when to introduce new stuff, but I do know that I had graduated to plasma weapons, and so maybe the game evolves with what the player unlocks and build. If that is the case, stay as basic as you can, for as long as you can