I seem to remember the ridiculous claims made when the MK23 was introduced to the effect of it being used to replace shoulder-fired weapons such as M4's and the like
I believe it was designed as a "ultimate" or "do-everything" pistol, which is a unrealistic concept, particularly for a sidearm. I don't know of any civilians who use one for self-defense (might be ok for home defense, where size isn't an issue) and the people who do have them are using them as range/safe queens. It can be difficult to find quality holsters that fit the MK23.