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Multifoil insulation (eg Triiso 10) supposedly has wonderful insulation properties (equivalent to 200mm rockwool or fiberglass) when lining a pitched roof. I am told it is not suitable for insulating an external wall. Why??
It would seem to me that if it is such a wonderful insulator then tacking it to battens on the (external solid) wall and then fixing the plasterboard over the top should be as effective as 200mm rockwool whilst taking up a minimum of space. The multifoil insulation would act as a vapour barrier behind the plasterboard and there would still be a small ventilated gap between insulation and outer wall.

Can anyone through any light on this issue?

the multifoil tends to create condensation more than fibreglass insulation.

Surely if it creates condensation then it is not an effective insulation. You only get condensation if the surface is cold.

thats why you use a breathable membrane on a roof,and with a ventilated soffit board condensation does not occur.
If you use it to line a wall the moisture is trapped and has no way to disperse, causing damp.