In no particular order:
I enjoyed those two episodes. I didn't know what does "filler" mean.
Filler means that it doesn't advance the plot. It can do other things, like develop the characters or the world, & thinking on it, the problem with the Great Divide is probably that it doesn't do anything beyond just filling space. I mean, technically we find out about these 2 tribes, but they're just such a minute, uninteresting detail that never comes up again.
There is also a separate but slightly related meaning of the word, usually in reference to anime, where it refers to sections that just plain weren't in the source material. Some people will try to tell you that this is the original meaning & the meaning above is incorrect. These people are wrong.
The reason why I hate the word filler is because too many people use it to mean "an episode I don't like." They don't seem the understand or care about the definition of the word. Like Mary Sue is "a character on the side of good, that I don't like."
I get that, but the problem is that this tosses out the areas in which "filler" or "Mary Sue" would be relevant criticisms, along with the ones in which they're just being used as buzzwords.
2: Nothing is wrong with The Drill, I love it, especially the sound design. But I'll also admit that in terms of the bigger story, it's not necessary. At the start of the episode, they want to go into Ba Sing Se, and at the end, they still want that. Nothing has changed, and I feel like that's why some people are down on it.
Yeah, I'm not really familiar with Drill-Hate. I mean, it's probably not my favorite episode, but it's okay & you could think of it as filler done right. It tells us what the Fire Nation has been working on behind the scenes, what their big strategy to win the war is, & is a big moment for the Gaang coming together as a team when they've been pretty dicey since the start of Book 2.
3: Not the machines per se, it's that a lot of the robots in LoK were CG animation rather than hand drawn, and it never looks quite right. This is most noticable in when Zhu Li and Bolin are fighting to escape from Kuvira. Mechs fighting mechs is fine, but the problem is that they look too light. That is a very common with CGI, you can also see it in, for instance, Pacific Rim 2 and Batman v. Superman.
The problem is that the criticism of the machines comes from a lot of sources. There's this, which I never really got but at least makes more sense than the luddites who complain about the presence of any technology while for some reason giving the first series a pass. But I myself complained about the Mecha-Tanks in Book 1, since they had impractical designs & didn't really offer a strategic advantage over the types of tanks that already existed. In fact, we later saw that they were defeated pretty easily.
This was mostly fixed in Book 4. It's true they had an easily exploited weakness, but EMPs seem to be a new technique that nobody had the time to develop protection against yet. That's a bit different. Then there's the Colossus, which is just all kinds of absurd. A big problem there is that bipedalism isn't actually very efficient, & that's why we don't tend to replicate ourselves when designing war machines.
4: UnaVaatu doesn't quite work because there was no precedent for that. Why is he suddenly turning giant after merging? Why is he changing his appearance? Why is he going to Republic City specifically?
Republic City was established as a place where tradition was especially flouted. I think Unalaq even complained about it. So him going there first makes sense, but the rest is basically it, the problem with Book 2's finale is that it does a lot of stuff without any explanation & is at times just plain inconsistent.
A channel on Youtube, Hello Future Me, once mentioned int a video (can't remember which one, sorry) how the LoK Book 2 finale simply doesn't work because there was no conflict being overcome that's represented in the character's actions, or earlier setup being paid off. Like in Book 1, the entire arc of Korra is working towards unlocking her airbending, which she finally does after she is completely out of options otherwise. That's payoff, but there is nothing of the sort in Book 2.
That was a really baffling observation, because it's made a point from literally the first scene that Korra's reliance on being the Avatar is "the problem." At the start of the series she's spamming the Avatar state, then in Civil War she's torn between what she thinks she's "supposed to do as the Avatar" vs what she actually wants to do, & finally in "Beginnings" losing her connection to her Avatar powers & losing her sense of self are entwined both metaphorically & literally. So I have no idea where he gets the idea that this conflict doesn't carry through the season.