How to accept causal claims when there is a lack of randomization and control?

After studying statistics, esspecially causal methods, I became very skeptical of any claims of causality without a proper experiment. I find myself not trusting any casual claim from observational research. I've read about how proposed mechanism or a multitude of observation studies can lead to a causal claim, but I find a lack of rigorous math to make believable. I've also read into some really interesting statistics about controlling variables, do-calculus, regression discontinuities, etc. Sadly, they all have major assumptions that don't hold.

I read up on Fisher's arguments regarding smoking and cancer, and his arguments are actually much more convincing than the opposing. When I look into other fields, like climate change, and ... let's just say I start to feel like a conspiracy nut.

There must be something I'm missing right?