One fairly common but low-key report during the 6.0-7.0 (and probably before) cycle is that rebel fighters don't show up very much, even on intensity 10.
I believe this is because, while their available strength "regenerates" fairly fast, they have some fairly persnickety rules about what range of human:AI strength-on-planet ratios and such indicate a time to get involved.
But it'd really help me to have some saves where you think a situation is going on (or can be speedily created upon loading the save and sending some ships on an attack or letting an impending wave hit, etc) where the rebel fighters should get involved but don't.
So if you wanna see more of these guys, donate saves to science