I don't have specific problem yet, I'm just worried about potential of having one.
When it comes to transactions, allowing bigger blocks means allowing bigger transactions (I don't know why it is so, but transactions are limited by block, not by their dedicated constant, possibly to potentially allow bigger custom operations). If there was an operation that slows drastically with size (f.e. some linear search in handling code, or a bug in JSON parser, etc.) then as long as blocks and therefore transactions are small, we won't see the problem. Once we allow big ones, the problem we already had, but hidden, suddenly appears, potentially endangering the network. I see similar potential with OBI, especially that at least for the moment it seems to me like it will make irreversibility a forking choice. Currently when new block is accepted, the block 15 signatures below becomes irreversible. Since it already had 14 signatures, it is highly unlikely it will not be irreversible in other forks, even if recently accepted block will end up being dropped. With OBI two different head blocks can become irreversible (I hope not, but I'm not yet convinced they can't) because they might receive enough votes each in different parts of the network, just some of those votes were revoked in the meantime.