I bought paid content for $5.99, and after playing for a while, I found that no matter how many times I played, I couldn't get new rare units, not even a wall. Is it because I've played it too many times in a short period of time, and the number of rare units available per unit of time has reached the limit? Or some weird bug?
Thats cause of limit of some rare units. I cant remember to own more than 3 Legends at once. Walls, infinyte fortresses and sanctuary beacons seems to ve unlimited. Arcane wraths, legends, tesla and so go on is limited in numbers. I suppose it is what you trouble with. If not, may be you farm not enough hard missions. Thor and valerian orbs start to drop from mission with Nikkis mega-craft. And one more thing: build more tech centers and update them. Every one of them have skill that grow chance to drop rare unit. When they not needed you always can store them in bank or just scrap