what makes the leveldb integration at boolberry different than what monero is doing? Or it is somewhat similar?
The leveldb here is only being used to store list of blocks, not all the other in-memory stuff that makes up a node (transactions, block index, output indexes, etc.). Monero moves all that stuff to the DB.
Advantages of BBR approach:
1. Simpler to implement and get right
2. Some operations still using the in-memory data may be more efficient
Disadvantages of BBR approach:
3. Higher remaining memory usage (but with BBR's pruning and lower volume of usage, probably not too bad)
4. Some possibility of inconsistencies between the two data stores in-mem and database (clintar's fix addresses one instance of this)
#1 is pretty significant IMO. Given that these projects are all works in progress, building the ultimate solution on the first go is not aways the best approach.
Is the other instance you refer to if the saved blockchain is more recent that the lmdb database (which I'm not sure how that would happen in practice unless you copied a newer blockchain.bin where there's an older db), or are you talking about something else?