Adjust register in games.

Especially considering that the C64 does not have a full circular scroll register either, it relies on a similarly limited range as the adjust register (0-7), after which you need to shift the name table, and yet smooth scrolling was more widely used.

I guess with the new VDP developers wanted to use the fancy bitmap modes where it is easier to make pretty graphics, but horizontal scrolling with the adjust register is more difficult.

Another reason could be that a number of titles were developed for both MSX2 and PC-88, which were limited by the latter system because of that.

Lastly, on C64 the same VDP was around throughout its entire lifecycle so developers could get intimitely familiar with it, whereas the MSX2 VDP was not around for so long so developers lacked experience and time to evolve their engines.

I think this could be the most valid explanation. But to me is was also a no real effort to push hardly the system as in other platform. I've always seen on msx some kind of small effort in trying to achieve the best from the machine.
They simply stucked with was available out of the box, so to achieve smoot scrolling they needed a full circular register available and easy to work with, no effort to look more far to achieve some more.

To be concise: the msx was not squezed like other platforms, a bit underused.

It's a thing that i've also seen on games in colecovision. With the same msx vdp found on msx1 the game quality was better than games only natively born on msx1

That's maybe a bit of a negative interpretation of the points that PingPong is trying to get across.
Sure, now we know all kinds of tricks to get the best out of 'obsolete' hardware. We basically have a magic book. It's probably wise to judge any software while regarding the time, place and circumstances in which it was created.
Just an example: compare NES games from 1992 to some NES games from 1985. It almost looks like an entirely different console. It's just the 'natural' course of software engineers, learning more about the platform as the years pass.
I think that some MSX developers tried to get the most out of it back in the day, while other developers just went for quick and dirty ports. So many different people, so many different approaches to creating something.
Just my 5 cents.

Of course it is an interpretation. As you can see i never mentioned the TMS VDP instead the V9938.
I partially agree with you. maybe it is the natural course ov events, but is the msx i also see some kind of lack of interest in trying to push its limits during the commercial era.
As we can clearly see to some very poorly done spectrum convertions, like one i've seen time ago (i do not remember what is the title) when in the main menu there was a option named "Kempton Joystick" in the MSX port.....

Horizontal smooth scrolling is also seen on games like Psycho World or Hydefos, before Space Manbow.