I think he's a good manager. I also thought Pochettino was a good manager.
We've had some questionable loans/purchases that I think RK or Erwin had a hand in -- Martina, Elia. But then I felt like we largely got Alderweireld because of RK. And Clasie (more later). OTOH, MP probably had a big say in bringing in Oswaldo and that was a disaster. Also, while the Black Box supposedly had already identified Tadic and Pelle, I think RK had some input/influence there.
I think MP did a better job of playing the young players. But I think that the impact of this might be overstated. Shaw was just good. I think Chambers to some extent as well. There's no doubt that MP still made a greater effort. I don't doubt his commitment.
Tactically, I would rate them about equally. MP stuck with one style, but he drilled the players on it well and it was well-organized and used to great effect. But it was one-dimensional. I think RK is more flexible, and allows players more freedom on the pitch. I think people tend to notice the supposed bad moves more than the good. For example, I don't feel like there were that many complaints prior to the Villa match as to the lineup. And he put on Juanmi and Romeu as subs and both played well and Romeu grabbed the goal for us. And yet, after the game there was much carping over RK's choices... and oddly complaints about Clasie being bad even though he didn't play.
Both seem to rank highly in man management as the players were very loyal to MP and him leaving may have hastened some departures to some extent. OTOH, it seems like Clasie loves RK and otherwise might not have come here. Elia certainly loved him.
Quite frankly, RK has had to deal with things MP never had to. He never saw Shaw, Lallana, Clyne, Schneiderlin, etc. poached because he left before his players did. And that has to be a big point in RK's favor for me. He came to a team in disarray and fixed it. And handled a second exodus quite well. MP quit. And did it in a bad way, IMO.
I like them both. But I also think that neither are irreplaceable. My opinion is that bad managers are really, really bad and can single-handedly wreck a team. But there are many managers who are good and somewhat interchangeable. It's kind of a matter of timing as far as having the right talent, and also having the right mix of tactic/personality with the squad.
Honestly, I think Adkins was better than both when he was here. Toward the end of his tenure, I basically for the first time ever had gotten to the point where I just gave him carte blanche and never questioned his moves. At the same time, it's kind of hard for me to see foreign players and real world class talents digging his Adkinisms or responding to his style. Or him being able to attract the same quality of players. So Adkins was the best during his time, but doubt he would be the best now.
As for the squad being weaker... yes, in my opinion it is. And no, Clasie is not Schneiderlin and that hurts. But let's be real. It's crazy to expect that anyone was going to come in and just step into Schneiderlin's shoes. We did extremely well just to replace Alderweireld with VVD. You lose Lallana, Lovren (he was good for us, after all), Alderweireld, Clyne, Chambers, Schneiderlin, an icon like Lambert, and Shaw, your first string GK and promising young striker gone through injuries... it is almost impossible to not get weaker. Even if you could somehow replace the talent, the massive amount of turnover in terms of players having to settle in and stuff is going to hurt you. It's frankly amazing that we aren't in a relegation battle right now.
I feel like it's good for managers to only stay 3-4 years. I think shake-ups help energize things, and players become better picking up different things from different people. Otherwise it all gets a bit stale. So if RK goes at the end of his contract, I'm okay with it despite liking him. I would be really nervous that we get one of the bad managers, but if we don't we will be fine.
The issues facing this club have to do with the business side of things. Can we generate enough revenue via marketing and the make right investments (in players or otherwise) to grow into a larger club? I don't know, but it's not really RK's purview. But that is what will make or break the club. For now, for the spending level/size we currently are, things are being extremely well-run top-to-bottom, and that includes the manager.
