When you start playing poker, you want to “play poker”. But, with experience, we quickly understand that a multitude of variants exist. Playing No Limit Hold’em is very different from Omaha, Stud or Razz.
Make sure your coach is good at the variation you are playing. Many people think they are strong in Omaha when they are not.
If you are aiming for a rarer variant, you can sometimes find good specialist coaches.
Think about the limits. If you play NL25, a coach who is very good at NL200 may be better than an NL1000 cash game player who might misunderstand the dynamics of low limits. Obviously, the coach would largely win, but he can underestimate the weakness of reflection of the opponents.
Then also be well aware of the differences between tournaments and cash games. A player like Elky is a tournament monster, but less proficient in cash games, and the reverse is true.