Only downside of the lens is 2300 gramms weight
Well the lens has got more downsides:
It extends while zooming - not optimal when you are in a hide
it lacks other "pro"-Features:
it has no AF-lock keys (Big glas and the 70-200 has, the 80-400 doesnt)
the lens shade mount has no lock
the tripod mount has just one thread (well the collar should be extended anyway=
given the weight it has no mount for a lens strap - it should have because it is too heavy to be carried hangig on the cameras bayonet (all third party replacement collars are without this feature as well) - i know that some people mount a strap on the tripod mount but I don' t like that approach
It weights 2300 gramms but the 200-400 has 3200 and is less compact.
NEVERTHELESS
I see it as a perfect combo between range, speed (fixed aperture), size, weight, image quality, (and price) often the #1 of my choice - and I like the soom mechanism , it goes smooth and usable (i didnt like that on the Sigma 150-600 SPORTS for instance) and it has one of the best VR modules (if needed, I switch it off most of the time) - perfect lens for handheld bird-in-flight shots