I think the limiter on smaller arms is quality servos with real location encoders - this one costs a couple hundred bucks for motors.
Not claiming the software is easy! But I think sourcing parts is (or has been) really hard.
Not a robot arm, but I worked on a project where the customer wanted to use a commercial motion platform as part of a simulator-based training system for boats. They thought they could just put it in the corner of their boat shed and get training but were amazed when they realised how dangerous it could be to passers-by, especially if it moved unpredictably when someone was standing nearby without paying attention. It went from 'we just need some crowd control barriers' to a full metal cage that was also integrated with the building fire alarm system so that it would stop cleanly if there was some sort of emergency elsewhere.
Other motion-platform-based hilarity ensued when it was discovered that the commercial software model they were using to drive the sim could in some circumstances capsize the virtual boat.
[1] https://www.youtube.com/watch?v=UerxNyu147g [2] https://mobile-aloha.github.io/
This typically won't be accurate enough for closed loop kinematics, especially at any sort of speed.
Many medium sized arms are quite capable of generating forces that can kill a person.