I don't know how many people are in my boat, but I like Tsukasa, but really dislike Tendou. That being said, I can understand why people hate on Tsukasa so much.
THE PROBLEM WITH TSUKASA/DECADE
It's quite simple, really. Decade as a show made the terrible decision to visit other rider worlds, but they did NOT get the original rider actors back. And further more, they made a habit of constantly having Decade one-up the previous riders. People did NOT like seeing Decade going around making fun of, beating up, or otherwise just showing up their favorite riders of years past. Even "AR" versions of them with slightly different names and completely different actors.
The worst victim of this was Kuuga, who was demoted from bad-ass rider who revived the whole KR franchise in the Heisei era, to Tsukasa's goofy and often ineffective sidekick.
Speaking for myself personally, I didn't watch Decade until recently, and I've watched very few of the early Heisei Rider series, so I was able to enjoy the show without feeling too much rage.
THE PROBLEM WITH TENDOU/KABUTO
This is going to need a bit more explaining. My problem with Tendou is that he feels like a Mary Sue. The writers of Kabuto seem to be in love with him, they write him as some kind of perfect superman and everything always works out for him because he's Tendou Souji. Maybe the audience wasn't supposed to take him seriously, but the writers seem to take him seriously. The plot virtually NEVER lets him lose at ANYTHING.
Take for example, when he first meets Drake. They get into a competition to see who can pick up the most girls. Drake is a womanizer and a flirt, and he manages to get a lot of girls. Tendou doesn't do anything, and shows up empty handed. However, when the girls Drake brought see Tendou, they immediately flock over to him, and he wins the contest. Why? Because he's Tendou Fuckin' Souji.
Yes, if we take a step back and look at it, we can think of it as silly and comedic. Why did Tendou manage to walk into a hospital and become a doctor instantly? Cause he's Tendou Souji. Why did Tendou become a high ranking police officer in a few days? Cause he's Tendou Souji. Why is he now a high ranking member of ZECT, a SECRET organization? Cause he's Tendou Souji.
But here's the problem. We can't take it all as a joke. The plot takes Tendou too seriously, because Tendou is the main rider. He gets all the power ups, he wins all the fights, he's the invincible superstar. The writers don't even explain half the power ups that show up, they just "appear" because he's Tendou Souji.
"The hyper zector has been destroyed."
"No, I have it right here."
"That's impossible, we all saw it blown up!"
"No, I am Tendou Souji. I was always meant to have this, so time travel/fate/destiny etc. It is now mine."
"That makes no sense!"
"You think you're so tough now cause you have that Hyper Zector, why don't you try fighting us without it."
"I have no reason to agree, but I will do so anyhow."
"Ha ha, we got you now!"
"No, you see, I also have this super powerful sword that just came out of the sky."
"Where the hell did that come from?"
"I'm Tendou Souji. I'm constantly evolving."
We are given a contrast with Kagami. Kagami, who is the everyman. The hard working guy who trys hard, wants to protect people. The guy who fails over and over but gets back up to try again. And this contrast of the two would be great if Kagami was the main rider and Tendou was the secondary rider. Having a jerk/too cool for school secondary rider is pretty standard for KR. But Tendou is the MAIN rider!
What does this mean for Kabuto as a whole? It means that Kagami NEVER gets to beat Tendou. We never get to see the triumph of hard work over the super boy who gets everything because he was born awesome. It means that Tendou gets all the new toys. The hyper zector, the big sword thingy. Kagami never got any power ups after he finally got to be Gattack. We should count ourselves lucky that Kagami didn't get killed, or written out of the plot, or ignored by the time the finale rolled around, like EVERY OTHER RIDER ON THE SHOW.
We even had a story arc with Tendou going around beating up all the other riders and taking away their devices, because he was that badass and no one else deserves to be a rider. And were we supposed to cheer for this guy? Is this really our main hero, the guy we want to save the day?