The speakers are screwed down well. The cab is built well. If it just falls over on the stage, yes...the jack is of concern as it's the most likely piece to be damaged.Sure they are. And, honestly, you're concerned about the JACK if the cabinet falls over?????
Also, as for power...I've used all the way up to 1000W amps with 1/4" jacks and NEVER had a problem with poor connection. You can transmit a LOT of power over a small connection point (which is much bigger than the pin-size that some people suggest).