It's possible that slow cookers are best for rich meat meals, but not for cheap or lean cuts. Lean cuts will get dry if you cook them in a slow cooker, so it's best to stay away from them.
1. Lean/expensive cuts of meat
Foods that are frozen should never be put in a slow cooker. Given that slow cookers work by cooking over very low heat, it would take a lot longer for frozen meat or veggies to cook properly.
2. Frozen foods
In the same way, slow cooking works great for softening tough root vegetables like carrots, parsnips, and turnips, but not for soft or green veggies. So stay away from soft vegetables.
3. Leafy vegetables
You might believe it is okay to add dairy or non-dairy products at the beginning, but you would be wrong. For best results, don't use milk, cream, yogurt, or even coconut milk in a slow cooker.
4. Creamy ingredients
Even though seafood tastes good, you should never put it in a slow cooker. This is because fish and shrimp are often very lean, and if you cook them in a slow cooker, they will get crunchy or dry.
5. Seafood
Slow cookers are great for making stews and other meals that need a lot of liquid, but you should never put pasta in them. This is because pasta can soak up a lot of water and get mushy if it's cooked too long.
6. Pasta
In the same way, you shouldn't cook rice in a slow cooker. The sticky rice will quickly soak up the liquid, making it a mushy mess unless you're making a type of rice soup.
7. Rice