I think the font size might be the problem.
Here is a picture of STM next to Unity Text.
- The font size is set to 32 on both of them.
- They are both using the Arial font.
- No AA or post processing.
- World Space UI
The text looks fine.
STM next to unity text but with Open Sans-Light instead:
With a thin font it looks fine too.
Further away from the camera and with a font size of 16 and thin font, however, it looks bad.
1. Unity text is naturally blurry. So I'd say that the Unity Text is normal.
2. STM is missing pixels, but not blurry.
Same distance as previous screenshot with 32 font size and Arial font:
Same distance as previous screenshot with 32 font size and Thin font:
I was going to try SDF, but that is very blurry on UGUI. ( Also as you said, it's not supported. )