Hi Willem -
I think:
Yes - fairly strict about developability.
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I believe this is offciially UnrollSrfUV now
This is an older (V4) and generally less accurate in some cases, I guess, UnrollSrf that had the advantage of maintaining the UVs of the input on the output - this makes it useful for creating base surfaces for FlowAlondSrf - some users were left hanging when this ability was removed in UnrollSrf so the older one was resuscitated as UnrollSrfUV.
Smash is the liberal version of UnrollSrf - it does not care if the surface is developable, it will make something regardless.
Different thing - it is designed to unroll somewhat double curved surfaces - like steel plates for a ship hull for example. It has some settings for taking into account stretch and compression. It works off of the render mesh of the object (and works on meshes).
-Pascal