Check out the SO100 arm, being supported by Huggingface and others. Only 4 axis, but cheap AF (<$250/arm) and using ML to make it more capable than 4 axis would seem. Also using identical arms for mirror teleportation/training.
Perhaps 4 is enough for any specific application, but then again perhaps 3 is or 2. :P
I've never programmed a robot arm, but I've spent a fair amount of time using a seven axis faro arm (a coordinate measuring device, sort of the opposite of a robot arm) and it certainly takes some practice to avoid "cant move there from here without reorienting everything", it's easy to take for granted what our brains do automatically for us. :)
There are a lot of downsides in 'arms' -- stacking all the axis hurts their speed, stiffnes, accuracy, increase complexity (e.g. in pathing, limiting speed, restricting to safe areas), limited reach.
I've always thought of them primarily as beneficial for flexibility at considerable cost, but if it's not 6dof then the flexibility isn't so great, then why not some other geometry?
Drone and helicopters has 4, and they are able to control max 4 of 6 parameters. Usually 3 positional + 1 rotational, and the rotatinal axis go first.
BTW, looks like it doesn't have closed loop control. "0.2mm repetability" (should be repeAtability) is only 'under certain conditions', no load.
https://www.youtube.com/watch?v=ECLrLupFW10
https://www.youtube.com/watch?v=_4mrb2T706s
https://www.youtube.com/watch?v=Ctb4s6fqnqo
https://github.com/adamb314/ServoProjectThe PAROL6 BOM doesn't provide any cost estimate, but it looks quite a bit more expensive.
Also they do measure position to achieve their feedback, might as well just output that on a 4th wire.
If you think about it, it actually makes a tiny bit of sense. First, it is failsafe: Breaking the control line or shorting it to ground will not move the servo to 0%, shorting it to signal level will not move it to 100% - it just doesn't move at all and stops applying force. Any sentient being within the movement range will definitely prefer it that way instead of random movements. Second, it can actually be pretty precise: The driver circuit can be completely analog, it doesn't have to be limited by arbitrary digital quantization steps. All it needs to do is check if the current encoder value is above or below the target and apply power to the motor accordingly.
So when you use hobby servos for robots, you're taking a low precision actuator meant to make a flap or throttle or other control surface go relatively up, down, in, or out, that was designed in like, the 70s, and asking it to do modern robotics stuff