I can only speak of my RIB, was fantastic for what I origianally bought her for, Cruising and fishing. The advantages where cheap to buy and run, fast but hard ride with a 80 2 stroke 36knotts. A wide platform for diving, fishing or whatever.
Disadvantages where, hard ride. Not to clever in a following Solent chop. Rubbing strake kept comming away from the bow (that could happen on any old RIB).
There where some stress cracks in the hull, but that didn't worry me as I trailer my boats so Osmosis didn't worry me. Was told that Stress cracks where common on BWM's and just the surface gel coat cracking and not to worry, my advice to you is to get a survey if there is any doubt.
My Transom knees suffered cracking after one particular trip when I launched Kohaku a number of feet into the air, that did concern me and was brought about probably because I had an overweight engine bolted to the transom. Had a proffessional repair done and the cracks turned out to be superfisial the transom was rock solid.
That's all I can really say. A great beginners RIB is the 17 and 18 but the 21 is a longer term prospect. Have a look at the Parker range of 21 footers, I think you'll find the design the same (or similar) build quality much more consistent and better tho. At least the design has stood the test of time and it's still in current production.
Looks Slow but is Fast
Member of the ebay Blue RIB cover club.