"Once Upon a Planet" is a nice sequel, and I like the fleshing out of the shore leave planet. But it's one of too great a number of TAS episodes that are derivative followups to TOS episodes. "Beyond the Farthest Star" is much more imaginative and intriguing. Gotta love that alien ship. Great Filmation art.
The main thing that bugs me about BtFS is the ubiquity of the life-support belts. It's a convenient plot point, having everyone in force-field belts so they won't be killed when the entity shoots phasers at people, but it's hard to reconcile with the rest of Trek, even the rest of TAS. And why would it be necessary for the whole crew to immediately go to the belts just because life support got shut down on their decks? It's not like the air would go anywhere in a hurry. They'd have hours before the carbon dioxide buildup became toxic or the waste heat levels grew too great. Plenty of time to evacuate to other sections.
The main thing that's bugged me about OUaP in the past is something I've always taken as a coloring error, the way the shadows of the robot thingies holding Uhura captive in the overhead shots were painted the same color as the robots themselves. But I just had an idea when rewatching the episode recently -- maybe those aren't meant to be shadows, but reflections in a shiny floor. Although the silhouetted figures of Uhura and later Kirk and Spock have neither shadows nor reflections. So it looks kind of odd either way.