[O]utside of the novel The Romulan Strategem, has Sela ever actually succeeded in any of her plans? (She did succeed in killing Rehaek, barely, but that wasn't her plan, it was someone else's.)
Yeah, but those times she was going against the main characters of which ever TV series and/or book series she was in and her plans were always MAJOR game changers so of course she was going to fail those times.
It's also worth mentioning the Romulan plans tend towards the game-changingly grandiose and ruthless in the first place, engaging in cunning and treacherous military maneuvers and sub rosa intelligence plots, testing and deploying superweapons, all with the goal of utterly transforming the balance of utterly crushing Romulus' enemies and establishing the Star Empire as the galaxy's dominant player. We've seen it on the TV shows, with the attempted invasion of Vulcan and the arming of Duras' side in the civil war; we've seen it in Nemesis
, with the deployment of the thalaron projector as a first strike weapon against Earth; we've seen it in Duaneverse novels, with mechanically-augmented telepathy and sunkiller bombs; et cetera.
With this sort of paradigm in intelligence and military affairs, would Sela's failures at making the radically game-changing success all aspire for necessarily mark her as incompetent? If anything, she did come quite close to success, and would have done quite well if not for our intrepid heroes.