Stack and reach should never be used alone. STA should be considered, in case it's too steep or slack and requires a seat post setback that's not available. Most frames in my size have a 74 or 74.5 degree STA and a common 25mm setback works. If the STA is 75, then I want a 32mm setback. I've seen brands with a 74 degree STA in all sizes and proprietary seat posts with only a 15mm setback. That's likely to cause problems for someone, but it must make the frame cheaper to build. If a round seatpost is used, some other brand of post could be used.
I've also noticed TREK models with their mast style frames having a limited saddle height range, even with two lengths of seat post to choose from. They list the maximum saddle rail height for the two post lengths.