I have watched the show from the beginning and season 1 and 2 were best of the 5 seasons. Karai is my favorite character along with Splinter and a few others. What ticks me off throughout the series was Karai's mutation was a waste. I think this was the first time an incarnation of her was ended up being mutated only the show's staff didn't do a good job with it later on down the road. When she got mutated it got me and other people talking about what will happen next with her. Whatever that unknown chemical was or perhaps an unfinished retro-mutagen that Stockman accidentally dropped in the serpent DNA that allowed Karai to almost revert back to her human self at the end of the episode got me thinking she was going to be sort of the 2012 version of Carter from the last 2 seasons of the 1987 series and later on in the 2012 series of she was going to help her turtles brothers or friends (which ether way you prefer) to fight Lord Dregg and his army of alien insects. After I saw the airing of "When Worlds Collide", I guess I was right though the 2-part episodes were not good just like the rest of season 5 and the first part of season 4 so far.
Season 3 started ok, there were some noticeable differences in the quality of the show, but I think it was the first few episodes after the Turtles returned to New York where the show was not getting better no more. I did like "Serpent Hunt" and its follow up episode "The Pig and the Rhino", however the whole Karai getting captured and having a brain worm in her head part was done awfully. "The Fourfold Trap" was good and I prefer this over "The Deadly Venom" episode as not only it was one of few episodes that she was in her full mutant snake form, but Splinter was there to not only save his sons and April, but his daughter as well, though the ending was the same of her not being completely reunited with her family despite she is now freed from the Shredder's control now. Let's not forget about the 2 part season 3 final. After the Kraang's 2nd invasion of Earth of the Season 2 final, did we really need a 3rd alien invasion in row to end the 3rd season like 1 & 2? To me it was a recycled plot from season 2 final and I was not in a mood for the Turtles to go space traveling because I wanted a good storyline for Karai and it was being delayed.
Since it was basically season 3 all over, the staff team were not taking Karai's mutant problems seriously. They could at least have her with the turtles, April, and Casey on the Ulixes prior before Earth's destruction. That way we could of see how she will deal with herself of being a mutant just like April was dealing with her psychic abilities and since April is also a mutant, the two could have a chat with each other, and if the writing is good as well then this could of made the space arc better. The only good thing of the space arc was that the cookie cutter effect for the areas were very limit as each location (excluding Dimension X and New York City) that the turtles and company visited were unique.
The 2012 City at War arc was very decent, but I won’t say it was great as season 1 and 2 and even though I liked the idea of Karai rebuilding the Foot Clan, I still felt that her mutant abilities were not used enough throughout the last half of the season and recently season 5. I understand she would like to be herself and I know she is not useless without her mutant abilities, but she could of easily knock off all of those Antraxas that piled up on her if she fulling changed into her snake form while they were on her in "The Insecta Trifecta" (I also thought Spider Bytes will going to be in that episode since I saw a Leo, Karai, and April in a spider web in the promo). You can also easily see there was no mention on Karai’s condition since season 3 and there is not even a scene where Splinter and Karai are talking about her being a mutant.
"The Forgotten Swordsman was the only good episode of season 5 so far for me... well... it'll probably be the only good one of that season as she puts her snake abilities to good use. Funny thigh is... even if she decided not to devour Tatsu and bite his head off and if he didn't got his soul taken by Kavaxas, he would of die eventually or quckilyfrom her venom since he took mutiple snake hands bites to the head (much worse than Leo taking one bite on both arms in "The Deadly Venom". In "When Worlds Collide" however, she did not use a single snake abilitlty. It is a shame that this show had became bad midway of season 3, worst in the in the first part of season 4, and not enough to fix it in the second part of season 4. The way season 5 is being made is a clear answer that the staff want this show to be over with so they can focus on the new TMNT series. This is why the plots since "End Times" felt like they been rushed.