The repair in supreme commander are ineffective. 75% is too expensive. For example, repair in Starcraft is only 25% and mechanics are very often used. The repair looks too bad compared to nano-repair or Air Staging Facility or regeneration aura. Why not buff it? At least up to 50%
You have to keep in mind that repair in faf works a lot different than in sc2 because reclaim exists. If you save a 10% hp ML and bring it back to your base instead of repairing it you can instead ctrl k it, reclaim it and rebuild a new one with a lot higher mass efficiency. In practice there is almost never a scenario where you repair for the efficiency right now. You mostly use it when you quickly need some extra hp on structures like when you're in danger of getting power sniped, but in such situations it does not matter whether the repair costs 25% or 200% of the original cost since you want to repair it anyway.
Ofcourse once you lower the repair cost some new scenario's might open up where you can use it but it is quite hard to properly balance it because different units will benefit a lot more from it. For example repairing a mega is 6,5 times as efficient as repairing a fatty. On some units, like on t1, a change like this will barely have an effect, but meanwhile for t3 navy it comes close to the threshold of making it viable to just repair your battleships.
Issues like this is why the repair cost was changed from 100% to 75% a few years ago and not any further. Yes we'd like it if repair was more viable in different scenario's and more commonly used, but balancing a core game mechanic like this is pretty tricky and will probably cost quite some time. Considering that there is already another feature (ctrl k) that mostly does the same job that people want repair to do and it applies to most scenario's the result was that repair isn't high on the priority list atm, especially since the balance team lacks manpower right now.
Repair is almost useless, should be fixed so that is viable, this is much more important in my mind than nerfing the mongoose again