I don't think the Lionel ones are that bad. They won't pull many cars but they all use can motors and worm gear drive, which makes it easy to use remote controls, and maintain reasonable speeds. My objections are a lack of weight and especially the lack of a flywheel, but see below.
Long before Lionel put Thomas in their catalog, a good friend used to make them by mating the ubiquitous ERTL toy with the 6-wheel chassis from a postwar 2037. Decent runner and very good puller! As with the Marx above, one advantage (compared to the current Lionel design) is that a child can push the loco by hand without fear of stripping the gears. I'm not sure if this should be encouraged. But if does happen, self-locking worm gears and rubber tires are a recipe for trouble!
