Correct terminology: "plain bearing" or "solid bearing".... all bearings are "friction" bearings, or more correctly, "anti-friction" bearings.
Anyway, back in the old days, the railroads had lots of yards, car inspectors, cabooses with multiple crewmembers, slow freight speeds (usually 50 mph or less), you get the picture. The plain bearing assembly is a maintenance-intensive item-- it needs to be oiled every few hundred miles, periodically repacked with the oil wick ("waste" used until about 1960, then the "journal pad" came along) cleaned or replaced every 36 months, the brass can get dislodged, etc. All in all, a lot of effort to keep 'em running right. There's no hazard, per se, but attention must be paid to the things, and when the yards, car knockers and cabooses went away, the ability to keep an eye on these old things did too. How many "old-timers" are still left on most railroads? Employees, that is.
Lots of ads in the 1950s RR trade press extolling the virtues of plain brgs, especially their lower first cost. Once Timken developed the AP ("all-purpose"-- and much less expensive) bearing about 1956, there was no looking back as more and more new freight cars were delivered on rollers.
Right from the start, 98% of road diesels, freight and passenger, rode on rollers. The slow-speed switchers stayed on plain brgs due to cost and speeds under 45-50 mph.
Plain bearings were prohibited on interchange revenue freight cars as of Jan. 1, 1994. There is NOT an "FRA ban" on them, but an AAR prohibition. A railroad can choose to move a properly service plain bearing with no FRA problems (for example, locomotives are not revenue freight cars!) but they have seized upon the 1994 interchange ban as an easy way to refuse to move "old, weird and historic" equipment.
You may have noticed that lineside hotbox detectors, originally developed in the late 50s to deal with plain brgs, are still used, and it is not unheard of for a roller bearing to fail, burn off the axle end, and cause a wreck or derailment (just one example: a NJ Transit MU train about 2-3 years ago). Would any railroad then say that roller bearings are inherently dangerous?
Properly serviced and lubricated, there is NO hazard to operation of plain bearings-- they DID work for almost 150 years, and at passenger train speeds (think 20th Century Ltd at 80+ mph!).
It's a skill kind of like firing a steam locomotive-- no Class 1 railroader would have a clue in 2004, but there are plenty of preservation railroaders who are real familiar with the things (steam locos and/or plain bearings)!!