A scorpion is powerful enough to do EVERYTHING you need it to. You can be conservative with it and play it as support and rack up a high kill streak and have a gunner for some extra support. It can also be used aggressively to catch up in a close game, and it can do that with only one person operating it; having a gunner would be too risky in that situation because of how exposed they are, and that risks deaths. The tank has been in the Halo universe for some time so it's comfortable for both noobs and veterans alike, but the Mantis is different.
The Mantis is solely a support vehicle. The turret on it isn't all that accurate and the rockets are miniscule and are only 5 rockets per reload. Not to mention that it becomes an instant target for all opposing players that spot it, so when that candle burns twice as hot, it lasts half as long, if not less. The Mantis's prescence on the battlefield itself is a huge psych out to the opposing team. If their Mantis driver was an idiot and got destroyed, there is a huge psychological advantage over the other team. That will lead to them scrambling and making mistakes, allowing the rest of the team to pick them off fairly easily. This is still only a support role and 4 out of 5 times, the Mantis driver is going to be stupid about it anyway, therefore, the Mantis is not overpowered.
As for the other vehicles in Halo 4, if you hate their durability, enlist in Operator and get wheelman. If they were any stronger, then those with wheelman would absolutely dominate every game and BTB wouldn't be any fun anymore. Vehicles are meant to be instant targets and taken out quickly. Besides, how doesn't love those epic explosions? The vehicles respawn at a fair rate, so it all evens out. If you hate the vehicles, go back to 4 v 4.
