FIXED in v5.65!
To me personally, the main problem that prevents REAPER's notation view from appearing "professional", is the awkward placement of stems.
At some zoom levels, stems are rendered a little too much to the left, so the stems are either displaced from the noteheads, or the notes look like lollipops:
Example 1:
Example 2:
A related niggle is the thickness of stems versus the thickness of the sides of the half note noteheads. At some zoom levels, the stems are noticeably thicker, so they cannot fit nicely together:
For comparison, an example from Sibelius: