Yeah, I generally find the threat of Seamus' gun is enough to force your opponent into awkward situations, and let you control the pace of the match pretty well. He's incredibly good at anti scheming as well as scheming with a combination of his gun and just being able to push people off of points with a combination of the daze trigger and terrorize. Seamus can just about guarantee any of the "Put scheme markers on your opponent's half of the board" schemes relatively easily, and usually with enough freedom to let him help out with the strategy as well.
Seamus usually frees up the rest of your crew to focus on whatever scheme he isn't doing, as well as play the strategy. I find I generally like to take him into Recover and Symbols pools, and he's not particularly bad at the other two. He's no Von Schtook, but I think he's always at least a consideration when the pool is relatively scheme marker heavy. I usually find I'm taking both the Rider and the Emissary with him, because he's generally too busy to want to play around with his crew mechanics all that often. They're nice when they happen, but I don't think he suffers playing with a more versatile crew, and Ressurs have some excellent versatile models.
If you do decide to take his keyword models, however, I think both Doxies and Belles are pretty reasonable for their cost. They have a ton of wounds and lure/take by the hand can occasionally be incredibly clutch. I'm a little more iffy on the Dandies, but only because of how card intensive Seamus can be himself. Setting a 7 aside for a 4 point model can be pretty rough, although the Dandy has put in work whenever I've managed to make room for it. Bete's an oddity in that she looks like she should be killing things, but I find she performs best as an additional scheme runner, as long as you have someone who can get corpse markers where you need them(Shoutouts to the Carrion Emissary). I've been generally unenthused with Sybelle, and I admittedly have not tried the Mourner since the beta, but overall, I think Seamus' keyword models perform adequately. I reach for most of them on occasion, when I think the scheme pool calls for them, and that's an alright place to be.
Overall, I don't think Seamus is going to be topping anybody's list as the best Ressur master, but I think he's a solid pick into a lot of scheme pools. He's definitely got game and I think if we see some of our better options get tuned down a little, he'll show up more. Besides, he's a blast to play.