They were a rage for a while especially families with kids but from what I saw over time was the kids would forget to take them, the parents would forget to turn them on, a lot of areas can be too noisy to hear enough conversation to be worthwhile.
From a mechanical point, remember that a ship is 80% metal, radio waves biggest nemesis. The communication equipment that is used by the ships crew is far superior to the low wattage FSR radios that can be purchased over the counter even if you purchase good ones which will run between $50-$75 a pair and they go through batteries fast so rechargeable are almost a must.
But with that in mind, they do work especially on the open decks but I have seen people bring them and not use them after the first day. Then there is the issue of taking them ashore with you. In some countries it can be illegal to use them, even the low grade output FSR radios so that is something that needs to be investigated well ahead of time if that is part of the plans.
Most now use the phone system and recording a mssg. on your home room that can be checked. This works with the older generation. Or stickies on the cabin door. lol
Some lines now have an onboard wifi system that you can use your phone just like at home. $$$ vary.