The engine is one of the smallest issues. I’d like to know more about how they plan to sustain human lives during a couple years of unshielded radiation and micro to macro particle bombardment, and zero gravity induced atrophy and bone depletion. Not to mention air and water recycling and food supply for a journey with no possibility of resupply. An inhabited transit to, landing, exploration, and return voyage through interplanetary space will require a lot more than a reliable engine.
How long has the space station been up??? Do you think that just perhaps, science has learned how to handle most of the problems you postulate??? In multiple ways??
Whats Space X’s Mission Success rate ? This is not a payload mission its a Human mission. Big difference. Not sure how any astronaunts would sign up for Space X.