Agree , just put higher price or longer time for higher gained stats.
more training with other kind of restrictions, expensive, longer time to refresh
The more training you add, the more advantage big teams will have. People who donate to one account (usually the accounts who wins every round) will only get stronger and more impossible to beat. Think about it for a second. Better to nerf the stats you get from assaults, making the stats you already have more valuable that way.