My DM kit-bashed the old beast companion rules with the UA templates:

Use the original creature's stat-block and normal companion rules, except as follows:
- calculate hit points with either beast of the air or beast of the earth method, depending on the base creature's type (land creatures use Beast of the Earth hit point method, while flying creatures use the Beast of the Air method. Aquatic creatures could technically use either).
- give the base creature the Ready Companion and Primal Rebirth features.
- Saving Throw DC's the base creature might have increase by 1 every time your proficiency bonus improves.

As written, the Beast of the Air/Earth gives the ranger only two options that are always the same (depending on their type). That's quite boring, because regardless of the form you choose for the beast, it always has the same statistics.

With the kit-bashed method, the already vast amount of creatures capable of being Beast Master's companions still remain valid, and besides, many of those creatures have unique abilities that are different from those two "Primal Beasts".