There were two answers to that question. One answer is that I am leery of anything which automatically converts my Java objects into a serialized format (bitten by Java's object serialization in the past). Incompatible object changes can be difficult or impossible to reconcile in a backward-compatible manner.
But the main answer I gave got some chuckles and further questions. I explained I was trying to avoid too much "pixie dust". In the example code, I was already using the Java Persistence API (JPA) and JAX-RS and their associated annotations. If I had not been careful, there would have been annotations for Spring and JAXB as well. All of these annotations are small in the code but have very large effects. Those innocent looking annotations subject my poor unsuspecting code to some very big (and some would argue bad) frameworks. Understanding how these frameworks interact is not only hard, but those interactions change as the frameworks change (possibly resulting in the system breaking with no code changes).
I have real misgivings about the number of annotation-based technologies that should be applied to any one project. Each annotation you use represents some amount of code you don't have to write. And that is, of course, a good thing from a development perspective. But every annotation you use represents 'pixie dust', behavior which is hidden from you and generally performed at runtime. That means that when things go wrong, there isn't any code to look at. It also means that small differences between configurations can produce large changes in behavior. That's a very bad thing for testing, production deployment, production maintenance, etc.
I've been thinking about this issue for some time*, so I was pleasantly surprised to find Stephen Schmidt's post admonishing us to Be careful with magical code. His post is not specific to annotations (he calls out aspect-oriented programming, for example - I agree that AOP is another kind of pixie dust). And he points out some examples of the "pixie dust" phenomenon. While I don't agree with his 'magic containment' example, it's a good post. You should read it.
As a rule of thumb, I think two kinds of pixie dust is the maximum to sprinkle on a project. So think hard and choose wisely when picking which ones to use: the more kinds of pixie dust you sprinkle, the harder it will be for you and others to understand and troubleshoot things, now and especially in the future.
*Thanks to Mike Easter for planting the idea of talking about the state of annotations in Java
