Polyurethane adhesive OK with biscuits?

No reference, but they will obviously work in the sense that the biscuits go in the slots, but they won't swell with the glue alone - dunking them in water will make them swell and help the glue go off. I've used them that way, with no joint problems, but I've never tested the relative strength with PVA/PU. If a biscuit joint is strong enough for the application, then I wouldn't worry about it.

I've PU with biscuits, edge jointing thick (50mm) boards. I did have broad, well matching jointed faces and lots of clamps - so optimum conditions for PU - the biscuits were more in the role of locators, and the biscuits already very firm in the slots. The previous posters suggestions to dampen the biscuits (and the whole glue face) sound good.