1992 6a12 transmissions - are they known to be crap?

Is it unheard of for something to brake going over a speed bump normally making the car as if it was in neutral forcing the driver to use his (well - practiced) ohshit - glide - to - the - side - of - the road maneuvre and for the car not to go back into any gear?
That's what happened to me the other day. only clue i have is that theres a relatively quiet buzzing noise when i shift the lever into L.
The people at the garage are telling me the labour costs for fixing it outweigh the price of a whole new transmission.
Does this sound familiar to anyone?