No, not at all. In fact, as I understand it, that's how the majority do it and our SIP supplier originally suggested. The oversheeting is a belt-and-braces approach... gives a small channel for any moisture than transfers through the SIP roof to wick away rather than remain stuck under the EPDM. I think this option, together with a good vapour control barrier internally, is probably overkill, but we plan to use this as a garden office for two people year round and with heavy usage, so it's a small additional cost / effort to play it safe.
If you don't do the oversheeting and 'just' do an AVCB, then I'm pretty sure it'll be fine for most uses...