I want to plant a boundary of thorny shrubs. Which such shrubs are easy
to
grow from cuttings? I've tried growing some thorny types before, but the
cuttings never made roots.

Pyrocanthus can be rooted from softwood cuttings and if you've ever "mixed
it" with such a hedge you'll know why it's called that - "Who planted that
flaming thing?!!" It grows fairly quickly too.

Geoff