In the UK, isolated transformers are how we allow shaver sockets in bathrooms. You touch one of the wires and (via wet feet a pipes) you create the path to ground, which is bad. But because the wire is only live when there's a complete circuit, it's safe. You'd have to grasp both sides of the circuit with different hands to cause major problems

A bonus for the shaver socket is that it is often switchable, so a 120V shaver can be plugged in. This is less of an issue now, with newer shavers having circuitry to allow them to run on anything from 100-240V.

But the ungrounded transformer prevents the fault currents that the RCD would detect, so unless I am mistaken there is no scenario where a RCD would provide additional protection beyond an ungrounded transformer.

You can feasibly combine an RCD and an isolated transformer, not sure I've seen this done before though or if it gives any real benefit.