"but I've read that some big Nikons aren't either."
What you have to understand, that at long distances, heat can play a big effect on the photo. I have a Nikon 800mm f5.6 Manual focus lens. Purchased it in 1990, and it's still as good as new.
I was shooting at the SUZUKA 8 Hour Motor Cycle Race in Japan in 1990, just a few months after I acquired the lens.
It wasn't until I got the Kodachrome KR64 trannies back, that some of the photos weren't any good due to heat distortion of the subject. It wasn't the lenses fault, but, heat waves coming up from the ground, actually softened the photo.
I was shooting in the middle of the day ( 38C or 100F )