KM is arguably the most difficult nation at higher tiers. It used to be at ALL BB tiers, but you have excellent BB2-3 nowadays. For the others though, it's very much about maximizing a benefit that many find miniscule, and thus requires a very skilled player to even see. That benefit is their range advantage, which is usually just a few millimeters at normal zoom.
It can be powerful if well used, but higher KM BBs are also cursed with poor durability, average to below average speed, and a combination of fewer guns, poorer shell damage, poorer armor penetration, and worse spread than almost all contemporaries, leaving them far inferior in both dealing and receiving damage to anyone else. KM ships are also usually huge targets.
To play KM effectively, you need to learn advanced techniques like slingshotting, as well as advanced situational awareness and a high level of patience. KM BB4/5/6 cannot really play that aggressively, as charging an opponent completely negates their only real advantage. You also need to learn AA. This is partially to blind opposing BBs, but as much because KM are popular bomber targets for enemy CVs (due to lack of durability) and often left to fend for themselves by friendly ones (because many think good AA makes fighter cover redundant). However, most KM BBs have do have excellent close to mid-range AA.
I have little experience with RM, but at present, I'd say it's much more user friendly. Lower angles at most BB tiers make aiming easier, and their high number of guns with good rate of fire help negate lower spread and individual shell damage. In general, many play much closer to the Andrea Doria, while the only KM ones that really do so are the O-Project II and the EBB3 based on it. RM supports a much more aggressive playing style where they can rush in and use the easier angles and high firepower to devastate almost anyone. However, there may be some more nerfing ahead for several ships, so I'm cautious.
For a new player, RM is probably going to be much easier, while anyone who learns to play KM well will find they can adapt to most other nations with ease.