We only use metal wire feed assemblies on our MIG welders. Plastic units are generally found on the cheaper DIY end of the market.
These give a much longer life span than plastic ones which wear out quickly and can struggle to give good consistent wire feed even from new and after a while can loose tension so the wire starts slipping on rollers giving inconsistent feed, so the weld bead will look erratic.
Metals feed units give very consistent wire feed speed which results in a lovely uniform weld, poor feed = poor weld.
All our feed units have reversable rollers to suit 2 sizes of wire, our MIG181 for example comes with a 0.6mm & 0.8mm roller so you can weld very thin material and thick material by just turning the roller around.
We have many customers who have struggled with a cheap DIY machines to get a decent consistent weld, they thought they were a bad welder, once they have have welded with an R-Tech machine, they comment that it's now obvious they had a bad welding machine!