Huh.
I've had a (mostly) very different experience.
I ordered the 6 oz. model, and installed it on a Savage Target Action - specifically from an early Palma rifle.
The trigger literally slipped on in a couple minutes - probably less. Pull the pin, slip the factory AT off (and keep track of all the loose pieces in the process), hook the new trigger on in the back, push the pin back in. Done.
According to my Lyman digital trigger pull gauge, it tripped at between 7.8 and 8.1 oz. Not 6 oz, true, but still pretty good.
Then the bummer - it sticks down and out further than the oem AT that the McM XIT was originally inletted for. So, the AT went back on for the time being.
Later, when I had some time - and worked up the courage - I fired up ye ole Dremel and got busy making things fit (in true Salvage tradition)

Once everything was fitted and working as planned, off to the range. No issues, and it worked just fine.
So other than some additional inletting work to make the McM XIT play nice with the trigger body, it went about as smoothly as I could have expected.
As far as the Stevens 200 actions that
@Rat Rifle had, the 2006 date is largely irrelevant. Those actions were basically a clone of the pre-AT actions - so pretty much anything that is supposed to work with the post-2005 AT actions isn't going to work with them. The bottom-bolt-release models... well I was never a fan of those to begin with, nor the cocking indicator inside the BAS, nor the smooth barrel nuts. The changes between the various actions certainly have been frustrating over the years.