I fee like you may be new to renegade, TC. I'll help you out.
All nod units are specialized, like the other guy said. You need to be a good judge of the situation and buy the right characters/vehicles. Flame tanks are very good against buildings, but just one tank won't accomplish much. If you have control of the field and you roll 2 or 3 flames into their base, there is a good chance that you'll get at least one building.
Stealth tanks are invisible when you're on GDI. They're only visible at close ranges or when fired upon. Stealth tanks are similar to the flame in the sense that you can sneak 2 or 3 into a base for some massive damage.
Tanks aren't always meant to be good against infantry with their main cannon, that's how the game is. You can still crush infantry, but they have a timed C4. Infantry are meant to be strong, that's why a single mid-tier infantry class costs as much as a tank.
Overall, you'll learn how to adapt to the strategy and weapons. It's a lot more teamwork and strategy-based than other FPS games.
Also, flame tanks will rip light armor to shreds, so if you can close the distance on an MRL, you can do some real damage.