there is no "catch." Sailing
RCCL in Europe is an excellent choice for value and there are few noticeable differences b/c you are in port a lot of days and super tired from touring when you come back. Think of it more like a floating hotel where you are gone all day (like marathon WDW days) with some entertainment vs. a Carrib. cruise where it is more like an all inclusive resort where daily activities are more important. We definitely notice the difference in the Carrib. and really miss the theater with 1st run movies for our teens, shows, and the rotation (and better, IMHO) dining on
DCL.
On RCCL, the entertainment shows do not compare with Broadway quality DCL, but who cares. Many nights we were too tired to even go--we just watched a movie on TV. RCCL also has better entertainment in Europe, as well--just not Broadway-typed shows (except on the bigger ships). We prefer DCL food, but if you commit to just pay extra for a few nights of specialty dining, that would be fine. RCCL MDR food is fine, just not memorable--reminds me of eating at Outback/Applebees. Standard RRCL rooms are smaller--but who cares. We sailed to the Baltics for less than 1/2 of the DCL price. That difference paid for us to stay for 1 week pre-cruise in Europe. I have also jumped ship to NCL (Epic) and although the bathrooms are weird on the Epic, that was also fine and 1/2 the price of DCL.
Interestingly, DCL for the Baltics and Norway next year were = or cheaper than other cruiselines for a family of 4--go figure.
I also suggest looking on trip reports here and on cruisecritic.com for the ports that you can DIY and save a lot of $$. Naples is one. It's easy (and safe, IMHO) to DIY from the port to Pompeii with a family of 4 for under $100 total with a little pre-research/planning.