I just had a very unfair battle (anarchid in enemy team) and had an idea how to limit such unbalanced games: add an elo rating to planet attacks, so if yours is much below the given one, you know not to join (would be futile), while the top players wouldn't defend against an assault that a weaker team could confidently deal with.
Though I'd be careful with hard limits (ie. prevent top players from defending against a low-elo assault or new players from defending against high-level assualts) - partly because in times with low player count a faction could find itself completely without noobs to defend against a low-elo assault, but also to prevent a meaninglessness of faction recruitment (if only battles with 45...55 win chance would be allowed, progress would be slow and independent of faction skill) - I think that some sort of soft 'evenness encouragement' would be useful:
a) stakes correspond to elo
A winning attacker can claim 'territory' on the target planet corresponding to the elo (or some function of it) of the defending players - winning against weak players doesn't give much influence, but defeating a top player team could instantly convert half the planet. To make attacks with low-elo players economical, I'd suggest to make dropship cost correspond to elo: transporting the common noob to another planet is cheap, but the deployment of high-elo players is rather expensive.
This would also go well story-wise with an Ashes of the Singularity-Posthuman themed planetwars champaign (more experienced minds need more bandwidth than new ones)
b) limit elo per planet instead of players
Weaker players can team up to take down stronger ones.
c) limit player influence
The more planets a certain player controls with his influence, the less new influence he gets per battle. Strategic aspect: bring good players to make sure you win, vs let others play it out to increase impact.
d) current version
Low-elo players hurt their faction by occupying battle slots; high-elo players need to fight as many battles as possible, since nonparticipation weakens their faction -> below average players should strategically refrain from fighting while good players need to spend as much time as possible fighting, since the campaign seems to be decided by total (high_elo_hours - low_elo_hours) per faction