If the arm is machined in the process to take a 'standard' joint, it will be weakened and may then break given a jolt from a pothole or lump of debris in the road.
If the bottom balljoint fails or the arm snaps then, with the steering arm at the back, castor action will make the wheel shoot outwards to full lock.
The result would look something like this - warning - graphic content, don't watch if you don't want to see such things.
https://www.youtube.com/watch?v=Uzq54UnwyKcThere aren't many components on a car that are especially safety critical, but the lower arm is probably THE most safety critical one - as critical as a wing breaking on an aircraft.
The examples you quote:
Failure of 1 brake - you still have 3 others (had this) - worst case you're still doing a lowish speed when you hit something, assuming you can't steer round.
Engine failure - Coast to a halt. Worst case would be failure while doing a dodgy overtake with little safety margin. There's a simple answer to that.
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