Just to preface this: This is going to be a mix of my personal views and feelings and my own analysis of the set. Expect a mix of objectivity and subjectivity.
I feel Ketar is one of the best sets design wise. He is the creature of stone and his color scheme reflects this. The various browns can be seen as sandstone, the lime greens can be compared to sulfur or similar elements, and the grey and silver can be seen as rocks and grey metals. Is his color scheme a little complex? Yes, but stone and earth is also pretty complex (stone comes from earth, so yeah). But I will be fair and say this will be a bit hit or miss for some people.
You could also make the argument that it would be easy for ketar to camofalgue himself given his color scheme in a stone ridden area.
I feel Ketar's theme was also a very good pick. Ketar is based off a scorpion and scorpions typically live in arid/desert/hot climates. The stone element is usually shone to be at home in the desert. So it would make sense that he would be based on a scorpion. And the scorpion theme is spot on even if he is missing half of his legs. This is barring his head for obvious reasons.
But the build has a lot to be desired. Note that I distinguish between build and design. A set can have a good design, but also have a poor build. I feel Toa Norik has a great design, but a poor build giving him limited articulation. And of course a set can have a good build, but poor design. My example would be all of the bohrok sets. They have a good build, but they are a bit simplistic and also clone sets. Not saying these sets are bad, I'm just saying.
Anyways: Ketar's build is questionable. The tail could easily be given more articulation and so could the legs. The legs could also be a little longer. The way his crystal swords work I find a bit janky as they move all over the place. Why this set did not come with 2- axle-1-hole thin connector is beyond me. It actually solves a lot of issues.
But yes, I feel Ketar's design is spot on while his build could easily be improved.
On a side note: I may be a little bit biased for several reasons. 1) I found the creatures adorable in the netflix special. 2) I tend to gravitate towards things that are typically disliked. See my liking of Solek and my liking of Trubbish, Garbodor, Lickiliky, and vinilite from Pokemon. But I still stand by what I said.