RE: brushless on Nimh
The other danger of running NIMH is killing your ESC. With the modern brushless ESCs, when you brake hard the current is reversed and it goes back into the battery. If that battery is too weak, the voltage shoots up and it can overvoltage the BEC, blow out the front-end capacitors, or cause failure any other number of ways.
The temperature thing is very true, I smoked my share of NIMH batteries also. If you are only getting 5 minute runs, that is 12C and too much for NIMH batteries. If you can't get at least a ten minute run (6C) you are probably not doing wonders for your batteries.