Firstly, I think we have to agree on terminology. A RIB is stricktly a Rigid-hulled Inflatable Boat. In other words it has a hard hull and is rather heavy! I think you are talking about what we call SIBs i.e. a fully inflatable boat, judging by your weight arguments.
In which case your point about tree-stumps, rocks etc are very valid-they would damage your boat (to an extent).
A RIB tends to be rather more resilient as "only" the GRP or fibre-glass would be impacted.
There are various ways in which this potential damage can be minimised however;
-try not to hit ANYTHING
-you should try to select a boat that has several chambers to the tube section. Thus if one gets punctured, the boat at least stays afloat (sort of!).
-try to run your outboard engine high when you reach shallow water-perhaps by not locking it down
I am sure others, more knowledgeable than I, will be along in a minute............
"Ribbing-the most expensive way of travelling third class"