I currently have in flight adjustable props on two of my three aircraft. Rearly could not see the need on a 912 powered Drifter as I can exceed Vne on ground adjustable in level flight!
I cannot think of a more cost effective solution to increase ones cruising speed in any aircraft. In UL/LSA aircraft around 10% increase the norm.
With Rotax engines a 914 over a 912 will not give this percentage increase (at least in the 914 aircraft I own), and cost over double the price. I think other solutions would also cost more, or impose cabin size issues.
Currently importing an amphibian into Australia and an inflight adjustable prop turns it from a 85kt to 95kt cruise aircraft which is even better. I have, and will, continue to recommend this option as one of the best to be fitted from the available list. I personally think more use than Dynons, auto pilot, or BRS if total price is of importance.
Pre jets, nearly all passenger aircraft post 1930's had adjustable props. The risks must have been less than the advantages.
As for problems, anything mechanical can break, however all in flight adjustables I know are designed to go to fine pitch which allows reduced speed flight and safe landings.