The average garden rhodie is not a great subject for bonsai for the reasons others have laid out. That said there are many many species worldwide and many hybrids as well. At the nursery I work at we sell a number of more compact types with smaller leaves: purple gem, myrtifolium and of course PJM. These have better attributes for bonsai.
The issue of backbudding is tricky. My experience pruning garden rhodies is that bare branches are unlikely to bud, but PJMs seem much more amenable to it. Still, it's maybe 50/50.