I am aware of the fact that this thread is six years old, but I found it while searching for something else. It is an interesting topic, and the comments provide some context for my own experience...
I had Jackie Brown build a flintlock fowling piece for me a number of years ago. I like the gun and Jackie did a good job with it overall, but I took it to the range the first time, and the trigger bound up after about four or five shots. You couldn't pull it, or get it to trip the sear. I removed the triggerguard, which was inletted and screwed in place, without pins, and removed the trigger assembly. This was an "early style" pigtail trigger mounted on a plate, but pinned through the "upward extension" or post on the trigger plate, as described above. Everything looked good, so I reassembled the gun and it worked fine, for four or five shots, when the trigger bound up again. I determined that there was not enough clearance between the decorative "web" on the back of the trigger and triggerguard bow, so I filed some off. It worked great for a few shots, and bound up again.
This time, I looked a little harder. It is noteworthy that Jackie used a wood screw to secure the tang, and not a through bolt. Also, the trigger plate was simply placed in the inlet, and held only by pressure from the trigger guard. I'll admit to being a little dull-witted at times, but I finally determined that the slot for the trigger and the inlet for the trigger plate were a bit sloppy. With the recoil of shooting, the trigger assembly was moving back just a little with every shot, until it would reach a point at which it bound up. Fortunately, Jackie had drilled a pilot hole for the tang screw all the way through, and it lined up with a "boss" on the front end of the trigger plate when the trigger assembly was in the right position. I simple drilled through the pilot hole and into this boss, tapped the hole in the metal, and installed a machine bolt as a tang screw. The trigger assembly didn't shift any more.
None of that would have happened if the trigger had been mounted on a pin through the stock in the first place. Not only is that more traditional than the "trigger group" you see so often these days, but it removes less wood, and the trigger's position relative to the sear is not going to change.
I recall that many years ago, Turner Kirkland promoted the use of this type of trigger assembly on his New Dixie Squirrel Rifle, as preferable to "a weak pin through the stock" (his words). I guess a lot of people took his word for it, or maybe the use of a "trigger group" is easier to manage in an assembly line situation, as for factory made guns. In my opinion, the pin through the stock is the better way to go, for traditional muzzle-loaders.
Notchy Bob