(1). Guaranteed compatibility with real Acorn style hardware (Virtual Acorn is a sophisticated emulation (in fact probably one of the best emulators around)) but *cannot* be guaranteed in all circumstances to behave identically to Acorn style hardware.
(2). You're running RISC OS (which has some bugs), on an emulator (which has bugs) on top of Windows (which has lots of bugs) is this a good thing or am I missing something ?
(3). No one can tell what directions Windows will be taken (and *if* VA will be able to keep up) Microsoft are committed to ".NET" and dropping a whole raft of WinAPI's. If *any* of this impacts on VA I can't imagine Bill Gates and his chums dropping everything to sort to problem out *just* so that VA can run.
That having been said at the moment (and sadly probably forever) Acorn users are going to be face to going to Windows for portables - which when you consider the power efficency of the ARM is a real pity...