Simply put, I haven't used PO in a long time, and so I won't be as familiar with it. PO also changes my IVs, natures, and other random shit, especially when teams are first imported (have no teams on PO).
In regards to battles being slow, that shouldn't matter for a tournament match. If you're worried about disconnecting, you can still reconnect. If you're worried about making a move and then it getting overwritten because you canceled a move and then picked another, just wait ten seconds and it will work.