Tendou and Tsukasa aren't bad characters, just characters who are poorly used as part sof their respective stories. Of course, most people just express their opinions from the gut, so they'll express this problem rather bluntly as, "Man, those guys sure suck!" When really, that's beside the point of the actual problem.
It is extremely difficult to consistently write a main protagonist who, intentionally, possesses noteworthy negative character traits. To do this, the rest of the story must be tightly structured around explaining why those negative traits must exist, and showing how those negative traits make for a better story. Both Kabuto and Decade suffer from loose, undisciplined story structure. Simply put, neither series ever demonstrates the dramatic necessity of their main characters' negative character traits. Without that argument to make Tendou's arrogance or Tsukasa's self-absorption feel essential, most people will react to them as if they were people they had just met.
And this means... well, people tend to come to the conclusion these guys are assholes who need to **** off, and they don't want to be around them. People don't actually give each other the benefit of the doubt all that much, when judging a new acquaintance. In fictional terms, this means they disconnect from the story and stop trying to sympathize with either Tendou or Tsukasa's point of view. This is not a failure of character, it simply indicates a failure of storytelling.
Tendou needed to be in a show with fewer secondary Riders, so his arrogance could be contrasted with the sheer necessity of his abilities. It's easier to begin viewing Tendou's character as unnecessary when he's surrounded by other Riders we're also supposed to like. It gets audiences to thinking that, hey, maybe that Gattack guy could just take care of all the problems? The story can try to say that's impossible, but once that thought crosses someone's mind, it's already too late. Someone who's had that thought is going to have a hard time accepting that Tendou must exist in the narrative at all.
Another problem dogging Tendou is that he was conceived as a parody of Toshiki Inoue, and then... Inoue was assigned to work on the show. And it's just painfully obvious in Inoue's episodes that he didn't quite understand who this character was supposed to be, or why he had to be that way. Inconsistent characterization hurts virtually any character concept, but it's harder on one that's trying to be complex and show that someone who possesses extraordinary traits isn't necessarily gonna be easy to be around.
Tsukasa simply suffered from the lack of vision behind Decade. Even in Aikawa's episodes, in retrospect, it's simply not clear what the show is about, what qualities Tsukasa should or shouldn't be developing as a character, or even what exactly Tsukasa is trying to accomplish with his world tour. The character feels passive even when the plot calls for him to behave otherwise, and this results in a confused and often inconsistent characterization. Often viewers simply reject poorly-constructed characters, the way they would reject real-life acquaintances who behaved erratically.
Masahiro Inoue's natural charisma is largely what makes Tsukasa likable, and likewise, Hiro Mizushima is often struggling against inconsistent scripting in his portrayal of Tendou. I feel like, if not for the actors in question behind these characters, nobody would really give a damn about either of them. And it's not because these characters were bad ideas, but because the ideas behind the characters simply were not executed well. A weak execution is going to make it more likely that individual members of an audience choose to reject the narrative, or in this case, reject its characters as part of rejecting the narrative's sense of priorities.