If the client knows the RPC password, they have full access to the node. Users should choose a strong RPC password, and refrain from changing the default setting that only allows RPC connections from localhost. A remote host would enable a MITM to execute arbitrary RPC commands. For multi-user services that use one or more zcashd instances on the backend, the parameters passed in by users should be controlled to prevent confused-deputy attacks which could spend from any keys held by that zcashd.
Users should be aware of new behavior in Zcash that differs significantly from Bitcoin: in the case of a block chain reorganization, Bitcoin's coinbase maturity rule helps ensure that any reorg shorter than the maturity interval will not invalidate any of the rolled-back transactions. However for Zcash, all JoinSplits which were anchored within the reorg interval will become invalid, rolling back transactions and reverting funds to the original owner. The transaction rebroadcast mechanism inherited from Bitcoin will not successfully rebroadcast transactions containing JoinSplits if the anchor needs to change—the JoinSplit creator must do that.
For receivers of funds from a JoinSplit, using a higher minconf (minimum number of confirmations) can help mitigate the risk of relying on funds received from transactions that may be rolled back.