I disagree. The shaft is of a harder material. These bushings are great for rotary motion, but have been used for years in linear motion applications even up to 4 m/s speeds....

These bushings will also lubricate as long as the bushing is there...

These bushings, unlike polymer, will hold there tolerances longer.

These bushings can often be found at local hardware stores (real ones, not orange box stores)

If It were me, I would upgrade the shafts to hardened and go with a sealed linear bearing...