No technology is 100% foolproof ; especially in a completely foreign environment ; and they don´t "crash all the time". According to Grusch I think they have recovered "in the double digits" which is less than a hundred. From the history of ufology we know about maybe 10-20 crashes which have lots of corroboration, thatś really not many.

edit: another pt to consider, if any of this turns out to be true - this is really insane, because most of us think "aliens" like one species.... but according to the many abductions and encounters in the past hundred years we may very well have been visited by TENS of different species... which may also explain the crashes, like it's not just one species/propulsion tech that is repeating their errors. Many different craft shapes too, point towards different tech, different points of failure.