Fantasy is only limited by the ambition of its authors. I think the genre is perfectly positioned to tackle moral and ethical issues.
I'd say the Pern series fits under Science Fiction, as would the first couple of Pendergast novels. Michael Moorcock has written quite a few novels that fit under Science Fantasy, including:
The Eternal Champion: The Eldren armies feature warriors called halflings. These halflings have the ability to teleport themselves by rearranging their atomic composition. Furthermore, the Eldren weaponry that Humanity refer to as "sorcery" are futuristic tanks, guns, and bombs.
The first Corum trilogy: Corum's race possess technology which allowed them to build a castle that travels between the planes and spheres of the Multiverse.
The City in the Autumn Stars: Libussa, the Duchess of Crete, constructs a human-sized crucible for use in an alchemic ritual. After her sacrificial death on a black cross during the Conjunction of the Spheres, she was reborn and intended to use the crucible and the Holy Grail to merge her being with von Bek. The result--a hermaphrodite--would be the ruler in the Age of Reason.
The Hawkmoon/Count Brass series: In the first four Hawkmoon books, Dorian Hawkmoon encounters a people with a device that can shift beings from one dimension to another. The Empire of Granbretan possess scientific knowledge which allowed them to create a mechanical throne-globe to sustain their King-Emperor Huon. Granbretan's armies are equipped with flamelances and ornithopters. The chief sorcerer of Granbretan helped construct the Empire's firepower and built a time-traveling device.
Jerry Cornelius: In the first novel, the character Miss Brunner has the tendency to absorb people. Brunner also spearheads a project to gather the world's brightest minds to construct a Messiah to the Age of Science. In the second novel, Jerry shifts through the Multiverse for a bit. In the third novel, there's a secretive gathering attended by many characters from Moorcock's work, including Elric, Erekose, and Hawkmoon. Plus lots of alternate history stuff, including a transmog machine Jerry uses in the second novel to turn people into whatever they desire. The main plot of the second novel also involves a box that increases the amount of entropy within the universe.
Science fantasy uses the trappings of science fiction--the technology, the alien races--and uses them as vehicles of character development. This is in contrast to science fiction, which can get caught up on "worldbuilding" with the futuristic technology etc at the expense of the story's characters. Of course, some science fantasy may also bring in more fantastic elements and use some real-world science to ground them or create a system for those elements.
On the subject of planets vs kingdoms, why not both?
If you're writing with the fantasy staples--elves, goblins, or whatever--study the folklore. There's many variations of elves and goblins in real world mythology. Thus, you could have an elf planet populated by the different kinds of elves from the various real-world cultures, a goblin planet populated by the redcaps, kobolds, and other different types of goblins from real-world cultures.