Stuart Noble wrote:
My gut feeling is that engineered wood should be more stable for an
exterior door than solid timber. The existing Victorian front door is
only being replaced because the bottom rail is ****ed, creating a gap
that varies with the seasons, and making any sort of draught exclusion
impossible.

Solve that by taking it off and thoroughly sealing it. And protect
against driving rain..



Also, I'm not totally convinced that an extra £55 for mortice and tenon
joints is really worth it.
The door will be painted, and is part protected by a porch. Aside from
the pros and cons (which I'm sure are exhaustive), has anyone actually
gone for dowelled, engineered hardwood and then regretted it?

If you mean simply using machined painted marine plywood, should be good
if well sealed.