I have used MK for well over 30 yrs now and yes it has changed somewhat especially the white or jet white as its called now,plus several of the metalics even got a roll of pink that was terrible to use.its much thicker and harder to shrink ,but heres something a lot of folks don't know if you are trying to use a heat gun made for UC on MK your waisting your time the UC is a low temp covering and the gun doesn't get as hot as the one thats made for MK.The end results are usually a burnt hole or not enough shrinkage .I wasn't aware of this myself until a few weeks ago when we compared 3 diff guns with mine being the oldest of the bunch over 20 yrs and still works great .It was also the hottest of the three .
As many planes as I build each year I got thru a lot of covering and yes I have used UC with good results ,until the sun hits it for a hour or so.reminds me of a prune and its back to the iron or gun to fix it back to the original condition ,if you don't pull it off the plane in doing so.Hint Iron works best *S*
Where as with MK I let the finished plane set out in the hot sun for a couple hours and let it get loose ,then a quick go over with the gun and I have a covering job that stays tight from then on .
Covering Material is a personal preference as to what a person uses ,whatever they can get the best job done is what they should go for.
The only time you can mix MK & UC is if your base is MK and the UC is trim otherwise you will pull the UC off the structure trying to shrink the MK.Another thing I have seen is MK doesn't like to stick all that well to UC .
Anyway just my 2 Happy Building.