The main problem with the ships (besides that they're just plain ol' ugly), is of course the scale issues. Of the six kitbashes, only the Constitution saucer and two nacelles are consistent with logical starship design.
Curry/Raging Queen: There's absolutely no good reason why the ships' nacelles were giganto-sized versions of the Connie nacelles.
Yeager: Upscaling the Maquis raider to double or triple its original size, and then slapping it onto an Intrepid saucer is just too ridiculous to take seriously, and trying to find logical reasons to justify this is really pulling strings.
Elkins: I can live with an Intrepid saucer with a secondary hull that came from an F-15, if only because the way it was kitbashed it looks nothing like an F-15. But the oversized runabout pylons look out of place, not to mention the Connie nacelles, which have absolutely no business being built onto a ship that advanced. Perhaps if the kitbasher had used the runabout nacelles to go along with the pylons, the design might be forgivable, but alas, he didn't. But then of course you'd have extremely upscaled nacelles again.
3-nacelled Excelsior: Although the only upscaling done was the use of the Connie pylons, the design is just, BAD. I mean come on, the pod that connects the upper nacelle pylons is hanging half off the ship!
Centaur: Although there's an upscaling issue with the Miranda weapons pod and connectors, the coolness of the design makes it forgivable