Traxxas's main issue IMO is their plastic driveshafts(this is an issue on the Brushless models) Basically due to the power they motors put down the drive shafts warp, and bend. There are aftermarket driveshafts, but they are like $50 a pair... for something that for how much is being charged should have been included on the damn trucks(note I own 3 traxxas vehicles). The other thing is alot of the claims about their durability come from people who swapped so many aftermarket parts onto the vehicles you almost can't call them traxxas anymore(seriously you can make their vehicles from 100% aftermarket parts).
The stampede 4X4 IMO is probably one of their best vehicles, but again the driveshafts warp. Other then that the truck is pretty tough, and can be quite fun. It is also an extremely versatile vehicle as with a few minor changes you can completely change its driving characteristics.
Their 1/16ths IMO stay away from those things are extremely flip happy due to the motor, ESC, and high Center of Gravity(this is especially true on the Summit). As you said he broke his collarbone I doubt you would want him bending over every minute flipping the truck back over. Also they need work right out of the box as the shock oil needs replacing right away, and getting new springs also would help (the stock ones make the trucks a pain to drive). I only ay this as a 1/16th revo owner. The 1/16ths are all essentially the same vehicle btw the summit just has bigger tires.
The ECX's from what I've read from many people that owned em I don't think I've seen any real complaints(well outside the very 1st batch a few years back that had bad radios). Plus for their price you could get 2, and race your kid