So, an anecdotal story for you.
For years, my main bike was a cyclocross bike: a size 51 Cannondale CAADX. It has a 371 mm reach and 536 mm stack. I run it with a 15mm spacer and 110mm stem. Been riding the same fit for years. It works well for me.
When I decided I wanted to dedicated road bike, I looked at geometries and did all of my comparisons on stack and reach. I bought at size 52 CAAD 12, which has a 535mm stack and 381 mm reach. Figured I’d run it with a 15mm spacer and 100mm stem. Perfect fit, right?
Not even close. I could never get comfortable on the CAAD12. When I took actual comparison measurements between the two bikes, it turns out that, due to head tube/seat tube angles, the bar stack and reach measurements were far off what I thought they were going to be. I don’t remember how off it was, but I know that the numbers made it perfectly clear why I was always sore from riding that bike.
The seat tube was also very long, so I had almost no seatpost showing. I have short legs, so I figure that’s more of a “me” issue that most other people won’t deal with. But it’s something I’m far more conscious about now.
Anyway, I finally had to get rid of the CAAD12 after messing with the fit for 8 months and still never getting where I wanted to be. I sold it and bought a 2018 SuperSix Evo in a 50 (same geometry as the CAAD12, but in a size smaller), which has a 526 mm stack and 375 reach. With a 25mm spacer and 110 mm stem, it fits like a glove–just a tiny bit lower and longer than the CAADX, which feels great.
So, all this is to say one thing: bike fit is about more than stack and reach. Top tube and seat tube length still very much matter in my opinion, as do head tube and seat tube angles. If you buy a new bike based purely off of stack and reach looking to replicate your current fit, there’s a chance you may hit the same issues I did. I learned a lot from that situation though, so now I know.