They will rust over time, because wood soaks up water and the screws will be wet wet over long periods. They should be OK if the wood doesn't get wet.

Ideally use stainless steel screws which are a bit expensive or galvanised which is a bit cheaper. I've also used solid brass screws on some nice bits of outdoor furniture - but brass is soft and you need to take care not to strip the slots or even over-tighten and tear the heads off.