I always thought the mouse itself was built like a tank, but I do remember having to regularly wiggle the connector in various directions until it settled in a position that continued to work.
They were actually notoriously unreliable. I went 6 months without one for lack of money to replace mine. Using the ST without a mouse involved holding the Alt and arrow keys down, and man did that suck.
Your comment brings back long buried memories of the time that my ST mouse broke and I had to save up for a new one. Hearing the ST keypress sound repeating as the mouse slowly moved across the screen, 8 pixels at a time, and then having to do SHIFT+ALT+ARROW when you wanted to position the mouse pointer with pixel-level accuracy...
