Which cast iron? There are many different grades/alloys with different properties. Sometimes cast iron (some specific grade) is better sometimes it is worse. And of course you can cast things other than iron, cast steel does exist (rare outside the feed to a press, roll, or some other process, but you can cast steel into a specific shape if you want to). There are trade offs. The topic is so broad we cannot even start to talk about it. Instead we start with what your application needs and then look at the options to get that.
Cast iron is fantastic for building machine out of - while it isn't as strong, it is stable against vibration. There is a reason engine cylinder selves are often cast iron.
> I had some crappy cast pewter toys as a kid
Those were pot metal, not pewter (pester has many definitions but implies some qualty control). They are made out of whatever melts in a pot - often whatever is cheap at the recycle yard (without trying to identify what is in the metal - including lead which shouldn't be used in toys). Typically no control of the alloy was made and often they start with several different things that are great in isolation but when mixed result in bad behavior. Then the next time the make the toy they use different mix and get different properties. If you spend a little extra to get a known alloy pot metal is a high quality castings with great properties.
> I’ve only seen the bargain bin results
You have likely seen a lot of non bargain bin results. However since the parts are invisible you never thought about it or the alloy used. You see the failures and so casting gets a bad reputation because it is obviously used in the cheapest things with low quality control. (the door knobs in your house are likely cast pot metal plated with brass, but they last for decades)