The issue for gunsmiths isn't how strong the epoxy filler is. It is a matter of how much it shrinks. That's why bedding compounds continue to sell well even in the face of so many other two-part epoxy compounds.
Here's the problem: You do a bedding job, get the action/barrel out and cleaned up and the bedding job looks great, right?
Look at how good it looks in a year or two... then decide whether you like that material as a bedding compound. What you'll find is many epoxies tend to shrink a bit over time, leaving you with a need to revisit the bedding job.
Marine-Tex shrinks the least of some of these materials, so it has found some favor.