I guess one factor I would check on is how flat/straight the stones are, and how well they lock together when stacked. If there is any wobble, it could get problematic towards the top of the stack. If that were the case, using construction adhesive between layers should help stabilize things as it creates a small bed between the layers to counteract any looseness or wobbles.
I will be watching, as this looks like a nice simple way to make gate columns. The approach I was going to take was to duplicate our porch columns, which means building up a concrete block structure, skinning it with mortar, and then gluing/mortaring on stone facing. Looks good in the end and simple enough, but still a lot more work than stacking pre-cast landscape blocks with similar net result. Here's a shot of the masons doing our porch 2 years ago -- I learned a lot watching these guys, enough to know I could do it but it would take time and patience:
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