I've been shooting a Marvel conversion in bullseye matches for the past three months. It definitely has the accuracy and reliability needed to win provided the shooter does his part.
As you noted, using the Marvel means having the same grip and the same trigger through an entire 2700. In my case, the consistency isn't perfect because I mount the conversion unit on my back-up .45 instead of on my Rock River pistol. (The slide stop hole in the RRA pistol is a bit too small for the Marvel's slide stop. A conventional slide stop can be modified to work but I haven't bothered.)
So why isn't it more popular? Well, for one thing, the Marvel unit is fairly new and many shooters have been put off the "one frame, two caliber" concept by the shortcomings of some of the earlier conversions (e.g., the Kart). The Marvel also costs more than a basic Ruger MkII or Browning Buckmark. More importantly, though, even the best 3.5 pound trigger is going to be harder to shoot than a comparable 2 pound trigger. And no pistol based on the 1911 is ever going to have the low bore line of a Pardini or an IZH. So the shooter who chooses to use a Marvel does make life a little tougher for himself. Finally, the Marvel might be a little hard to use with a frame-mounted dot sight -- I use slide mounts so that's not an issue for me.
