It's all very well saying that something is "boring", but if they can't say why it's "boring" other than the fact that certain Infantry match-ups vs Tanks are unfavourable for the Infantry then they may as well be stating the obvious; that's the way the game is inherently designed, with counters in mind.
It's not an unknown point that basic infantry don't do well against tanks. They didn't do well in Renegade, they still don't do well in Renegade X (even with C4) and they don't do well in other games in the FPS genre; for example, try taking on a tank in Battlefield using a class that isn't the Engineer and you're going to have a bad time.
I can see that the idea of "more variety = better" sounds good in theory, but that isn't always the case. More variety adds more complexity to the game, which in turn drives up the skill ceiling and makes the game harder to grasp for newer players. It's very important that when you are considering to add something to an established game ecosystem that you introduce features that have strong reasoning behind their inclusion.
On the subject of APB though, I think the main difference here is that APB isn't trying to be Renegade, whereas Renegade X IS. Obviously you're going to enjoy Renegade X more if you prefer Renegade, but the fact of the matter is the choices made in APB are made in order to give infantry clearly defined roles that have strengths and weaknesses, which makes team composition a LOT more important in rushes and such.
In Renegade X, there's not much consequence for picking a single type of infantry class and rolling with it, because:
Every infantry has C4, which means they can combat vehicles to a certain extent and attack structures effectively (this is true with Renegade as well).
Every infantry has the option to buy a repair gun, which really de-values the role of the Engineer classes.
Every infantry has the option to buy additional weapons, which conversely powers up Engineer classes and allows them to do fill in the gameplay roles of other classes (which, in-turn, de-values other classes).
It's almost as if there's an active discouragement in infantry unit diversity because the intention is to let all of the classes buy weapons to fill most of the gameplay roles, apart from more specialist units such as Snipers, the SBH, Anti-Tank specialists and the more expensive versions of them. There are no truly specialist infantry classes in RenX (like the Spy or Thief), who can do unique interesting gameplay things that place them outside of the arbitrary roles of: "Anti-Infantry", "Anti-Tank", "Generalist" or "Engineer". Every character class has a gun that is good at shooting a certain type of thing (which is inherited from Renegade) and the buy-able weapons blur the lines across those four basic class categories even more. Basically, if you make everyone special, then nobody is special.
I'm not sure what they mean when they say "Renegade X has more features". In terms of vehicles, APB has a LOT more. Granted Renegade X just introduced that commander view feature which seems cool, but I couldn't get it to work properly last time I played, so I can't form an opinion on it. It's very hard to say that one game has more features than another without establishing a baseline of what a feature is and then counting them up. Though both games have a good amount of features, I'd say.
tl;dr - Variety is the spice of life, but it's not always the answer.
Personally I think the reason that APB has fairly low player-counts is just a problem of exposure. There's not enough people talking about the game outside of the community to generate more interest. It's not a problem with the game itself, because it is really fun to play.