Botulism toxin poisoning from canned foods is comparatively rare. Problem is, it only takes once. Botulism spores are everywhere. They're in soil, on vegetation, in our guts. The spores aren't the problem. It's the toxin they produce when they live in an anaerobic (airless) environment. That's home canning.
If steps aren't taken to kill the spores either with high sugar or acid content or by pressure canning to a temperature long enough to sterilize the food, then botulism spores may live in your jars and produce toxin. The toxin kills by shutting down muscle movement. Not a fun way to go.
Home canning with oil, or canning very dense foods such as whole garlic cloves or pumpkin, are of particular risk. Oil added to food makes it very hard to get a good seal on the jars. Dense foods can't be heated to their interior to high enough temperatures even with pressure canning to ensure all botulism spores are killed. Consuming such home canned foods is a form of Russian Roulette. Lots of people will say things like, "Well, that's how my family always did it and they're fine!" I have no doubt that is true. Equally true is that they are taking an enormous, unnecessary risk.
Different people are comfortable with varying levels of risk. Me, I won't take the chance. If I can't boil it hard for 10 minutes to neutralize the toxin, then I follow my grandparents' excellent practice: After profusely thanking the giver for their kind generosity and after the givers leave, into the trash it goes.
It's a little bit like someone handing you a gun. You always assume it's loaded, even if you're pretty sure it's not.
Madness takes its toll. Please have exact change ready!