Of coarse there have been mental breakdowns from DMT, you don't just hear about it a lot since it isn't a popular street drug, where mushrooms, depending where you live and who you know can be.
Some have smoked it and haven't been able to get a good grip on reality for months. It is a very difficult experience to integrate and when it gets in the hand on someone who doesn't know what he/she's dealing with it can get ugly.
I've read countless reports of people freaking out and being scared shitless after smoking DMT claiming they will never do it again. Same thing goes with ayahuasca too, I actually personally know people who have done it just once and said they are never doing it again because it left them traumatized.
DMT is serious business, and sometimes it doesn't really matter how much psychedelic experience you've got beforehand, nothing can prepare you for a high dose of DMT. Nothing.
There are however people that can handle it, and those are the people that most commonly invest their time and effort in it to extract it, or to brew their own brews. A small percentage of people has the mental capacity to be able to appreciate this kind of experience.