Really? I thought I was having allucinations. 2 years playing this game and there are still things that surprise me, apparently. How does it work? Do you have to deplete everything else's budget or what?
There's a unit on every wave that has a respawn priority if other units can't spawn for some reason.
For example, w7 Cerberus, when you kill all 6 Dragoons, the game will start spawning Engineers. If there's less than 80 budget available and you just killed a Phantom, it will spawn an Engineer in its stead (or 2? I don't know precisely how that works). W9 Cerberus unit that fills this role is apparently Guardian. To actually spawn additional Guardians beyond the first 2, you'd probably need to kill a Phantom with less than 80 budget remaining or an Atlas with less than 100 budget remaining.
Two were one shotted with the same Talon headshot from full health... Is that enough? Otherwise I might have to get back and kill them moar 
No... your computer has been bad, so punish it accordingly.