The particular plan that took him down, the attempt to acquire Dominion technology and shuttle it through the wormhole to the Typhon Pact, did seem to be reasonably well-constructed--the planners thought the Breen merchanter had the cargo capacity to transport the material, they expected the Dominion couldn't intercept the ship's transmissions or detect it, they thought that the warbird could sneak out of the Gamma Quadrant past DS9. It's just that things went wrong that the planners couldn't control, whether we're talking of the Pact underestimating Dominion capacities or of the timing problems with DS9's bombs. It's easy to imagine circumstances where the plan did go off successfully.
Actually I thought that the plan only really fell apart when they decided to add the bombs as a contingency plan, I mean if their hadn't been any bombs the crew wouldn't have found them, the station wouldn't have been on alert, and the warbird could have gotten away before either the Enterprise or Robinson could warn them about the deception.
know that, but the Romulans and allies who planned the backup plan turned out to make the wrong assumptions. The communications jammer that the Romulans had placed near the Gamma Quadrant terminus of the wormhole probably would have been enough--neither the Odyssey
nor the Enterprise
beat the Romulan vessel back to the Bajoran system, and without communications no one would have learned what happened in time to prevent it. Coming up with another backup contingency plan to ensure that DS9 couldn't intervene would have been only prudent. Imagining the plan backfiring so spectacularly was something no one involved imagined.
Romulan intelligence underestimated the Dominion (fooled by the Breen acquaintance?) and overestimated the Federation. I don't think that the case can be made that this was a product of Romulan incompetence. It just was bad luck for Sela and Tomalak that things happened the way they did in the order they did.