Most metal shim gaskets seal the water and oil ways mainly by means of the viton or flourine rubber coating which is sprayed on in the form of micro balloons. These compress and merge to form a seal on the head and block faces. It's this black or dark coating that seals on the machined faces of the head and block where any imperfections lie (machined finish = hills and valleys, to same extent). In use this coating is compressed and physically moved around to fill said hills and valleys. On an engine with a good few heat cycles this coating will bond to the head and / or the block. Re using a gasket of this type is risking imperfect water and oil way sealing. The combustion chamber seal is made by an embossed ring or rings in the thin shims. The clamping loads reduce or even remove the embossing, so localised clamping loads around the combustion chambers are reduced, possibly fatally.
Spraying Hylomar on them is NOT as good as the original viton or flourine rubber coating, nor will it be as even. Personally I think it's madness to reuse them on an expensive and hard to work on engine for the cost of a new one especially a turbo charged one with high cylinder pressures and high thermal loadings.
Cometic say you shouldn't re use them, Reinz say you shouldn't, and FelPro say you shouldn't. People do re use them, for sure, some get away with it, same as they re use old valves in new engine builds, don't life valve springs and re use stretch type head bolts.
I re-used 1, I split into it's 'shim' layers, polished them all clean, then sprayed Loctite-3020 on both sides of all shims before re-riviting and fitting.
I had no problems with this.
I wouldn't do it again though as it took ages to do well.