A ribbon has a distinct sound compared to a dome tweeter. The low moving mass of the ribbon compared to a dome give them an "airy" sound (sorry for the audiophile mumbo jumbo). The Fountek as well as the Arum Cantus and Raven are all excellent ribbon tweeters.
On the negatives, ribbons tend to have worse off axis response than a dome which makes good off axis driver integration and power response harder to achieve. Also ribbons are far less forgiving of being overdriven or driven too low compared to a dome tweeter, in other words easier to make them distort and blow. Both of these characteristics mean that ribbons require a lot more skill and care in the design of their crossover than a dome tweeter. Textbook formulas or off the shelf crossovers need not apply.
Speakers with ribbon tweeters can sound very good, but it takes a skilled XO designer to make this happen.