The plastic from a Sabot or the plastic ring on the Power Belt doesn't seem to cause much of an issue as far a barrel fouling - at least in the field/range. Like anything (even a lead bullet), a good cleaning after a day of shooting will usually clean all that crap out of the rifling.
A few years back a few of us got together and did some (sorta) scientific tests to see what the different powders would do to steel (and the pellets or sticks are just formed powder, so whether it's a 777 pellet or loose 777 it shouldn't make much of a difference). We tested 777, Pyrodex, Shockey's Gold, Blackhorn 209 and Goex (what we collectively had available).
We took some steel plates, sanded off one side (to clean away any contamination) and touched off about 10 grains of powder (small pile - each powder got their own plate so there was no cross contamination) and then just left it out in the elements for a week to see what would happen.
There was "no sign" of rust on the 777 plate.
There was "some rust" on the 209 plate.
The Shockeys and Pyrodex plates were clearly rusty, but only "slightly less" than the Goex.
Our conclusion was, that anything other than 777 WILL Rust your barrel if you fail to clean it after shooting - regardless of the claims they make.
I shoot loose powder myself and either Goex (first choice) or Pyrodex Select, but I do shoot side hammer guns with number 11 caps so need something that's "easy to ignite" - the 209 and 777 are the most difficult to ignite of the powders we tested.
Even if I could shoot pellets I would opt for loose powder - because, you can infinitely tailor your load - with pellets you are limited to 50 and 30 grain pellets - so could shoot 50, 60, 80, 90, 100, 110 - but what if you get your best groups with 95 grains? - you would never know if you limit yourself to pellets (plus the 30 grain pellets are usually more expensive then the 50's, which are expensive enough - a pound of powder will give you about 70 X 100 grain loads
Yes, it is "slightly more of a hassle" to shoot loose. But I simply measure at home and dump the charge into a speed loader (which also holds the bullet and the cap) - and just carry that into the field/range when shooting.
A quick check at Cabela's (just to get a retail price) shows a box of 100 50 grain 777 pellets is $50 - that will give you 50, 100 grain "loads" (or a buck a shot).
A pound of 777 is $45 and will give you 70, 100 grain loads (64 cents a shot) - so if you shoot a lot it could add up - but maybe you consider the convenience of pellets "worth" the premium.