I'll start by saying I do not own the LC thomas, or any of the other, similarly designed low-end LionChief sets like the 'Little Steam' or dinosaur engines, which appear to be cosmetically different, at least with the style of remote compared to the more expensive sets such as the Polar Express, which I do own. I have, however put the question out there several times asking for information about failures of LionChief products and have tried to get as much data as possible from forum members on there experiences. There have been the occasional failure that required service from Lionel, however with the number of these engines out there, the 3 or 4 major failures reported wouldn't scare me off.
The biggest issue I would look for are reports of the speed knob being broken off on the remote. I do not know if the knob is designed differently in the lower-end sets, but in the sets and LC+ engines I have, the knob seems to be designed well enough, and should not break unless someone really cranks hard on it past the stops. I suppose it could be broken by a rough child, but I don't find it likely.
I've also seen issues reported with the Thomas especially of DOA units, so I would test the engine as soon as possible, preferably in the shop before you buy it, if you are going through a local hobby shop.
Hopefully this is of some help.
JGL