The newer batteries are not the problem and neither are the speed controls.
One important point here is efficiency, which magnet DC motors are not too good at.
The equipment is failing because it's being pushed beyond it's limits. The motors are demanding a lot of current, which the batteries happen to be able to supply. A big portion of this energy being wasted by the motor, again, efficiency. It's not the batteries fault, it only supplies what the motor asks of it to cope with the load it carries. The load is just too much for either the motor or speedo, so one of them goes poof! Speaking of load brings us also to weight, which TC have a lot of IMO.
Brushless motors are much more efficient, but are being overloaded and the speedo overheats. With these motors, the stress carried by the comm and brushes in brushed motors is now carried by the speed control. And we all know pretty these come out after a run.
Not to mention this brushless high load scenario doesn't have too much time under it's belt.
Brushed motors need to be more efficient, by a lot. This is easier said than done when you have to keep cost in mind.
Compared to brushed systems, Brushless systems...well they are only in diapers, they just need more time to develop.