I've seen other people's campaigns implode over half-orcs, paladins, rogues, assassins, and more. I've seen Kender played just fine in a number of campaigns. If Kender collapse a campaign, the reason is bad players and bad DM'ing, not because of something inherently unplayable about Kender.
You can think they are unbelievable, just like I think a great many things in D&D are unbelieveable, but they are playable.
I still don't think they are playable if you play them the way described in the book. All of the times I saw them ruin campaigns had nothing to do with bad DMing. The DM ran the game just fine. But the time our Kender decided to steal the entire silverware collection from the Duke who invited us to dinner...you know WHILE we were eating the dinner had nothing to do with bad DMing. The Duke rightfully got rather annoyed that his expensive silverware was being stolen and tried to get the Kender to put it back. The Kender responded, as recommended by the book, that the silverware in this pack certainly LOOKED like the silverware that used to be on the table a second ago, but that cutlery had been in his bag before he got there.
The DM, rightfully, then had us all arrested for obviously being thieves who tried to take advantage of his hospitality to rob him. Which derailed the campaign fairly heavily as we spent the next couple of sessions trying to escape from prison with various players getting very angry at the player of the Kender for doing something so stupid. With the player of the Kender continually defending himself as just playing a Kender the way the book told him to.
The player certainly could have chose not to steal that silverware. Which would have removed the problem. However, literally the ONLY defining characteristic of Kender is that they are Kleptopmanics with no fear. Refusing to steal the silverware because he was afraid of what might happen would mean he wasn't fearless. Failing to take them simply because he decided not to take them doesn't make him much of a kleptomaniac.
The book literally says to steal almost everything you see all the time without fear of consequences. It says you literally have no fear of death. That means that each and every common sense thing humans do in their daily lives to avoid death Kenders just don't do. They are also super curious to try every new thing they can think of. Which means they all should be dead before they are even adults due to being run over by carts, jumping off cliffs, eating toxic things, and just seeing how it feels to be stabbed in the head by swords. Their "god" is the personification of their desire to put themselves purposefully in harms way with no thought to how it will affect them. How is that a believable race in any way?
I understand that some classes can cause problems with campaigns. However, it is possible to play an "assassin with a heart of gold" or a moderate paladin. However, even given that, I agree that classes are a bit more of a problem. Classes imply a behavior and a personality to go with the class. And behavior can be a problem. The behavior of a paladin could cause problems in a campaign.
Races don't imply any personality or behavior that would cause any problems with a campaign, though. If someone picks elf you aren't going to think, even for a second: "Here we go. He decided to be an elf. Looks like we're in for a campaign of stealing from all the other party members and running head first into danger." At worst, you might worry about the elf's vanity or racism getting slightly in the way. Most races aren't defined by their personality traits. They are defined by their physical traits with minor personality traits. Elves have pointy ears, the ability to see in the dark and are very dexterous. They are known to be slightly inclined to appreciate nature, magic, and have been known to be haughty. But all of those personality traits don't apply to every elf.
Kender is the only race that is defined WAY more by their personality traits than by their physical ones. Worse yet, they are defined by problematic personality traits.
My point all along is that when the book says "you are a curious kleptomaniac without fear", playing that gets you killed or causes problems with the campaign. I agree that it is possible for someone to decide that their kleptomania is mild and they avoid stealing things whenever it would cause problems for the party, they avoid stealing valuable things or things other people would get angry at them for. You can downplay their curiosity to avoid them running into dangerous situations to make sure the other party members don't get mad at you for it and your character stays alive. You can try to rationalize a fear response from your character by claiming that he "just doesn't feel like dying yet"(which was the excuse given by Tass repeatedly).
But all of those behaviors are an attempt to make sense of what is written in the book by downplaying what is written there to the point that Kender AREN'T an absurd race that should have killed themselves off ages ago.