What reasons are there to avoid vertical through-hole resistors?
Mounting resistors vertically is something that you can find even in high-quality old equipment.

This photo shows the main PCB of a Fluke 27 DMM. It was taken by Dave Jones of EEVBlog. The date code on the ICs suggests it was made in 2004/5.


These were designed to for industrial use. (e.g. in mines with potentially explosive atmospheres and by military forces) it seems that vertical mounting isn't necessarily unable to withstand a fair amount of vibration and shock. I have a more than twenty year old ex-military Fluke 25 that carries many scars of rough usage and still works very well. There are plenty of these around so I don't think it's an exception.

Here's a photo of the main PCB in my Fluke 25. (Date codes on the IC's suggest it was made in 1988, the PCB is marked 1984 design).


Not a lot changed in 17+ years.