Nylon shield bearings are okay as long as you don't try to eliminate the grease. Here's the problem----the degreasers, at least those in my experience, cause the seals to swell and make them drag a bit. That's why the metal shielded bearings. You can clean those.

I keep some metal shield bearings, good ones like from Boca, for racing. The rest of the time I just use $1 ones from places like Avid and throw them out when they get rough. Those are nylon shielded