The biggest problem people have with BDB, to my understanding, is its licensing. IIRC, it's free for non-commercial use, but otherwise you need to negotiate a commercial license. It's also owned by Oracle now, and I haven't ever seen any example prices. Having to suddenly negotiate with Oracle once you get profitability makes some people uneasy.
(I don't really use Ruby, so I don't have any Ruby-specific alternatives to suggest.)