Some people use them behind a stout motor combo and the last...sometimes they don't. Not trying to be vague or trolling. If you're going to use it as a drag car with slicks,high rpm launches,and a stout motor...not going to stay together very long. Use a less aggressive clutch and you'll have it last longer. If it's an occasional drag toy/mostly street-hard street they are a good transmission option. If you're making more than 500-600 ft lbs torque...you should look at a tko600. Same rules apply but they seem to be a little more durable.
Here's the deal: as gears pass power under load, the try to move the center of the gears way from each other. That force is absorbed by the transmission case. As long as the transmission case holds the gear centers where the should be, everything is fine. When the gear centers start to move away from the optimum position, it moves the load on the gear teeth from the middle of the gear teeth to the tips of the gear teeth. The tips of the gear teeth cannot carry the load, so they wear excessively and eventually fail. The T5 gear case is much lighter and thinner that the gear case on a Tremec 3550, TKO 500 or TKO 600.
The3550, TKO 500 or TKO 600 transmissions have a heavier and stronger gear case. The gears do not move apart and it keeps the gears meshing at their design placement. They put the load at the middle of the gear teeth and it does not shift out to the tips like it tends to do with a T5.