I think it because people associate anything gaming as low quality (which can sometime be true).We've had militaries killing each other for over a decade using PS4 and Xbox controllers why is everyone getting hung up on this one.
If their compliants where focused on the fact that it is from an older generation or that it is a 3rd party one (TBF some people have picked up on this) then they would have a point.
It is false to assume that a custom controller would be more reliable than an off the shelf gaming controller. Low volume stuff can have a lot part to part variability. However the actual components that make up the controller like buttons and joysticks will probably come from the exact same factory that spits out the components for gaming controllers anyway.
Also gaming controllers (at least the first party ones) are quite reliable. Sony Sold 106 million PS4. That's at least 106 million PS4 controllers that exist. Assuming people only played their PS4 for 20 hours, that is 214 million hours of real world testing. A 1% failure rate is 1 million unhappy users.