I thought that foaming polyurethane glues were not very good for external
joinery .. great for tight fitting joints and fast cures, but strength not
brilliant ? ... or at least that is what I read here.

Very waterproof, and capable of strong joints - but must be well
matching surfaces and firmly clamped. If it foams and gap fills
because there's space in the joint (rather than the external foaming
of excess glue being pushed out of a tight joint), then not so great.