I think it's a low priority issue.
but, we should know the possibility of an unintended result.

_exists() function always executes before create() or save() function in Model.

_exists() function => use read connection.
save() function => use write connection.

There may be long latency to replicate between master and slave servers.
We can't be guaranteed that slave servers always are same as master server.
It could be a bit problematic in high transaction.

In my opinion, you'd better use write connection intead of read connection in _exists() function.