Using an ND filter to get a slower shutter speed is the way to go. The more expensive cameras usually have a ND filter built in. Shooting at 24fps is possible on most modern cameras these days.
If you want to have the ability to adjust the amount of blur in post shooting at 120fps+ allows for that workflow but it’s a pain to do.
It's more to do with the way motion is captured by the sensor than shutter rates, ND or applying motion blur. Someone else posted an article about global-vs-rolling shutter, which gets to the heart of that jittery video look common to CMOS sensors found on DSLRs.
Also, in film, people generally keep the shutter angle at 180 degrees. So for 24 frames per second, the shutter speed is usually 1/48 of a second.
As for codecs, that's actually where some of these cameras are making leaps and bounds. The GH5 I mentioned is capable of 10 bit recording, which means you could record ProRes HQ with an external recorder.
If you want to have the ability to adjust the amount of blur in post shooting at 120fps+ allows for that workflow but it’s a pain to do.
Or is it something else like a bad video codec?