I have a small horizontal set up in a positive pressure cabinet for multiple units. Worked great for my original S9/L3 Setup of about 12 units.
I originally went this design for space. I had hoped they would also run cooler than horizontal but saw very little cooling improvement.

As I rotated units to S17 Pro's,I (and Whatsminers) I found Horizontal NOT to be a good idea for the S17's.
Despite their bad design an inevitable failure, I believe Horizontal led to premature failure as a number of heatsinks either eventually fell off or sometimes just slide down out of place leading to overheat and shutdown. A few times I was able to re-heat the sink into place and get running again, but eventually it would fail again. Of my mix of units, my 3 S17 Pros (with 9 boards), have had 8 of 9 boards fail. Some have since been repaired, and 2 deemed unrepairable. I suspect if I had kept Horizontal, some of the failure may have been at least delayed.

Now I am working on transition to S19's. With the improved heatsink I hope the vertical orientation will work better. I only have one S19 95Ths (non pro) right now. They seem to run quite cool compared to previous units. They appear to report outlet temp vs chip temp. but I can feel the cooler exhaust of the unit compared to S17 or M20/M30 Whatsminers.

I am continuing with the horizontal set up, especially now that the S19 (as the Whatsminers) have single, large, secured with screw, heatsinks.