marlin60,
There are two variations worthy of discussion, in the J frame line. My preference are the shrouded hammer units are the Bodyguard variations, the hammer is shrouded but still accessible (for function testing and DA shooting if necessary).
The Centennial variations have the enclosed/internal hammers making the gun DAO. Some of the model numbers for this style are M38/M638/M49/649/438.
Here's the Bodyguard style:


Some of the model numbers for the Centennial style are M40/M42/M640/M642/M340/M940 (I'm sure there are others in the newer metal series - Ti/ScTi)
Here's the Centennial style:


You ask about disadvantages, from a shooting standpoint, I don't see any for either variation. From a functional variation, I prefer the Bodyguard since you can perform a function check withou the use of the trigger.
Both are great guns. I point out both versions since the factory has recently released a new Bodyguard version, the M438.

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.............SmithNut