All nu retro game fans turn to youtube talking heads to learn everything they know about obscure collectible consoles like the "stuck-in-time" PC Engine. Thankfully, there is a never ending stream of experts who make rarely known facts easily digestible by sprinkling them throughout hilarious comedy.
If you are dead inside and can't appreciate humor, you can jump straight to the first sentence of the actual game/console commentary at 3:45
Part 2 opens with another technical explanation of why the SNES Drac XX looks so good compared to the NES-looking Rondo of Blood.
Later they explain why the PCE game doesn't have any parallax in it. What inspired them to explain it, was playing through a section of the SNES game featuring some very plain horizontal parallax, the kind found in many NES games. They point out how that section has so much 3D depth and how the PCE game doing the exact same thing more times than the SNES game does, is so flat and primitive and non-existent. To be fair though, that SNES game section immediately followed a parallax laden recreation of the crumbling bridge section of the PC Engine game... only with less parallax. They then enter the main areas which make up the bulk of that stage in the SNES version (which features completely flat parallax-free backgrounds).
In the end, to prove their hardcority and biaslessness, PCE Drac X gets a 9 while SNES Drac XX gets a 7 ("too much platforming").
This is why youtube personalities are the go-to source for retro gaming info.