Soap and water, and that is genuinely all. FRN is glass-reinforced nylon, injection molded, waterproof, chemically resistant and completely unaffected by humidity. It needs no oiling, no sealing and no special treatment, which is a large part of why so many working folders use it.
FRN does not absorb water, does not rot, does not corrode and does not care about temperature swings. It is the most forgiving handle material in common use.
Because the texture is molded in rather than cut afterwards, it also does not wear smooth the way some machined textures do.
Molded grip patterns can be intricate, so a brush works better than a cloth. Warm soapy water and an old toothbrush cleans anything ordinary.
It resists solvents well, though keep it away from prolonged heat, which can distort a molded part.
Very little. FRN flexes rather than shattering, so it survives drops better than a laminate. Heat is the main enemy: a knife left on a dashboard in summer sun, or run through a dishwasher, can warp.
The usual criticism is that it feels less premium, not that it fails.