I actually think they're pretty good honestly.
The first one's a little hokey in having a villain who's "not really a villain, just going through a bad time" more or less, but I did enjoy his motivation, of being unhinged because his cutie mark got mutilated.>>36825557
I liked 1 and 3, but found the second to be the weakest of them. it was basically just a blatant last crusade rip off, and not much more to it.
it also SERIOUSLY showed how terrible of a villain Azuihotl is. He's basically a giant raging Child. He has brute strength in comparison to a pony, but in the actual context of the world, his brute strength is small shit. Dragons and other bigger things easily outrank him, and he has no magic that I can see, which easily trumps him.
He's also INCREDIBLY stupid, behaving self destructively and only avoiding death due to having someone to actually do the thinking for him along for the ride. Not to mention he lacks the charisma of dr. cabelleron.
the third was slower paced, but made up for it by setting up an arc and world building. I enjoyed how they were going for an overarching meta-arc of multiple artifacts needing to be Found. Hilariously, the MANE SIX seem like small shit compared to the cabal of magical unicorns doing shit in the world, given how little they've done (apart from twilight) since losing the elements.
On a random note, is it just me, or do the books try and tease daring do and cabelleron as a couple with some of the flirting remarks? it kind of comes off as them trying to portray cabelleron less as a villain (with Azuihotl and other ponies instead taking that role) with him being more the "antagonist that ends up teaming up with the hero every now and then" thing since the book mentions daring do and him having worked together in bad circumstances befor.