Well, speaking only for myself, a couple thought come to mind:
1) There are a lot of unfounded keyboard opinions available in this less scientific of shooting hobbies called muzzleloading;
2) Manufacturers / builders selling to the public seem to be understandably over cautious from a liability point of view;
3) And others should only do what they personally understand and are comfortable with;
That said, I personally approach such things with a bit of research / planning / thought and had no worries about excess pressures out of my modern high quality .40cal GM barrel.
For example, double balls loads out of MLs are a well established fact of the muzzleloader world...and a pair of them is usually equal to a similar weight conical that's offered in a given caliber. In this case a pair of 92grn .395" balls is a 184grn payload...essentially the same as the 200grn REAL.
In addition, I used 2F instead of 3F for a beneficial element regarding the pressure curve.
In addition, I used a mid range powder charge of only 70grns Goex 2F.
Both double ball loads, and 'oldnamvet's REAL, are extremely accurate for me, and neither give any unusual recoil or indications of excess pressure whatsoever. In fact, taking that beautiful 8 pointer with the 200grn REAL in the shoulder at only 30 yards didn't even make a pass through.
But this is what "I" did...I make no recommendations that anyone else do anything I do.
My standard recommendation is that everybody should follow manufacturer's guidelines, period.