I have Annie and Clarabel; I've had no problems on curves and they generally go through R1 points OK although I have had the occasional derailment. However, that has usually been down the to grandsons's having disturbed the wheels in some way. The axles pop out quite easily, but they also pop back easily. Also, one of my R1s is on a 1 in 30 slope (it forms a temporary loop during line extension) and that does give some issues when approaching from the diverging route; but then it does that for a number of other things too and I can't really say I'm surprised; it will be going when the expansion presently under way is complete.
My two (currently aged 6 and 2) have not been able to do any serious damage to them in more than a year of strenous play. The couplers, however, do damage fairly easily but luckily I have quite a few spare from other Bachmann items. If I run out, it will be Kadees; even the boys couldn't damage them.