Hey, Glasgow tiler I have been able to try on a small area where I needed to build up by 25mm or so, the clips were fine but I agree they are not for everyone, on large commercial work completely impractical on a financial basis.
One thing to add I have been able to use the wedges three times over so far but I think that probably about it with them as the serrated top edges are showing wear now.
Again I would like to stress so far only tried on porcelain so can not give a view on other tiles i.e travertine etc.
For me the pegs are not there to level the floor they lessen the chance of lippage when the adhesive is drying back. The clips will not pull a tile up or down by huge amounts so the adhesive contact should not be broken as the clips will break if you squeeze the wedge in with to much force