The Warlord can enable the Rogue to sneak attack twice per turn at will is the issue. Everything else that allows that is limited in some way ergo the Rogue is not supposed to have the opportunity to sneak attack twice per turn at will.
But... so what?!
The rogue SA was gained early and scales as they level.
A warlords "give someone else an action" could be just the same thing... gained at whatever low to mid-level and it basically "scales" as the other character's "actions" scale as well.
How is it more imbalancing for the warlord to get a class feature usable each round which can let a rogue get a new sneak attack each turn than say a second rogue getting their own sneak attack usable at the same rate?
it seems that almost by definition the warlord's ability is an inestimably balanced element since its exact power depends on how powerful the other characters are.
Again, within the concept of "is this a balanceable class feature" i do not see the problem.
Any character can use the HELP action to give someone advantage on an attack, is that "imbalanced" when the sneak attack it helps hit is doing 10d6 (there it might even be enabling the sneak, not just Helping it.)
A mastermind rogue added to the party instead of a warlord can use help out to 30' as a bonus action and then also have its own full sneak attack available... GMs in terror of an extra sneak attack from warlord-assisted rogue should really be trembling at the idea of the mastermind added to the mix instead of a warlord.
obviously, any new class can be overpowered and overloaded, but when looking at one feature, one which enables others to do more of the stuff they were doing seems almost completely one balanceable in design by its very nature.