I absolutely disagree.
First: the Warden "only" gives an additional attack to Guards. To make them unkillable you only need a generic hero... a starpriest will give them the bonus to save, and the Starlight bonus. Guards are a tough nut because you can easily give them a save +2 rerollable, and for that you don't need the Warden. The warden is gold for the Selfless Protector ability, that keeps alive your most precious character.
Second: when you need the Warden, Kroak (or any Slann), can summon the Warden at 5+, and that's only a minimal fraction of the things they can do in the Hero Phase. The key is all here: if you have a wizard, that can summon the model "A" without breaking a sweat... the wizard is more useful then "A" alone, because since the first turn, my army will have the same synergies guaranteed by "A", and will also have the support of said wizard.
So yeah, I think it's definitely better to have Kroak that, during the first turn, can summon that precious Warden, can also summon one Bastiladon (buffed with Spirit Shield), then casts a Comet.
Now, the Warden is exceptional... but he's not in the same league of a Slann, or Kroak. And that's why they cost more points.
It's not a matter of power Vs synergy. It's a matter of power+synergies+support Vs synergy.