If the pipe work is plastered into the wall and not put into some kind of channel or in conduit from which it can be withdrawn for inspection then it contravenes the water regulations.
Copper pipe can be obtained which is coated with castellated plastic coating, this protects it from the corrosive effect of cement and allows some thermal movement. It is intended for use in underfloor heating though.
You should form a duct for the pipework and protect it from cement by using Denso tape or similar.
Burying fittings (tee's, elbows etc) is not a good idea. If they decide to leak you will have to dig out to rectify.
If your house has solid brick walls then there is the risk of freezing if the hidden pipe work is not insulated.
If the pipe bursts or leaks the wall will become saturated and the tiles will fall off.
In the shower area, you are best advised to chisel out a chase for the pipe work if the inner skin of a cavity wall is concrete / cement block, insulate it and tile over it.
If the wall is solid brick, dry lining can be used to hide the pipes. Use cement board in the shower or other wetted areas.
