I think it's possible to be objective. I've worked on a lot of 1970s gear, and usually the integrated amps have some features which the receivers from the same manufacturers do not. Typically separates employ features not found on receivers, for example:
- Better quality sealed volume controls
- More substantial heat sinks
- More output transistors
- Defeatable tone controls
- More robust chassis
- Better layout, for better s/n ratio and channel separation
- Larger power transformer
- Larger filter capacitors.
If you look closely at any brand, be it Yamaha, Pioneer, Sansui, etc.. the extra money for the integrated or separate amp did buy something tangible. I know a lot of people like receivers, and a lot of receivers are great, but there's some limits when you try and cram everything into one box.