The coefficient of friction between the steel and/or brass axle ends (journals) and cylindrical brass bearings is greater, probably by a significant degree, than that between the same axle ends and the ubiquitous engineering plastic side frames that we mostly deal with these days. Also, it is pretty uncommon these days to have good wheel sets that have cylindrical rather than conical axle ends.
The Tichy nylon bearing inserts are pretty good, and are designed to offer better rolling and wearing characteristics in truck frames made of soft styrene. The downside is that such use commonly results in either trucks that with the inserts are too narrow to accommodate almost any commonly available axle sets, or if they do fit, the truck is far too wide to begin with.
Athabaska made some very fine low profile conical brass bearing inserts designed for styrene E&B passenger trucks, if I recall; and Reboxx also once made some excellent engineering plastic bearing inserts.
All in all, IMHO bearing inserts have little application in the present world of current freight and passenger trucks, but have found limited use with imported brass trucks, and the few trucks still being made of soft plastic.
Denny S. Anspach MD