I long had the opinion that flexible glues would be better for brittle wood such as snake wood or ebonies, in addition to some bone-tooth-mammoth tusk. Brittle woods and materials still contract and expand with large temp and or humidity swings, which in my opinion contribute to the cause of some woods/material cracking.
Rigid glues HOLD and don't allow for the contraction and expansion. Flexible glues do. I'm not sure how urethane glues do but urethane glues expand with thousands of tiny bubbles. It might have a tad bit of flexibility due to the bubbles. I started using rubber epoxy when I first saw it in Japan when I lived there. It is hard to find it here (USA). Then there is the flexible CA. Today's flexible CA is different from the black flexible CA that first came out.
This brings us to your question - I think flexible glues should be good for gluing blanks into tubes.
How well has it worked for you?