Have come to the same conclusion as Maurice. 0.5mm seems to be the thickest card that it will cut with no difficulty. The problem seems to be not so much the depth as the density.
This is rather flimsy soft 1mm card whgcih it cut through pretty easily. However this card is IMHO too spongey for modelling but the denser 1mm card I usually use and cut manually won't work.
This is 0.5mm and cut easily.
I think this will save me heaps of time and frustration for the cutting of repetitive parts, especially wheels where the part can be painted/coloured later. Track parts usually have to be cut with more precision as part of the 3D appearance is generally in the printing...