Are the chances of losing a thumb with hook grip really high? If not, can other problems occur, and which would be the most relevant?
The hookgrip hurts, but it's not dangerous.
I'm not sure how you envision yourself losing a thumb. The roundness of the bar is far too dull to actually cut the thumb off.
If you're worried about the restriction of blood flow, this is also not a real threat.
In some orthopedic surgeries, tourniquets are applied to restrict bloodflow, to prevent bleeding during the surgery. A time limit of 1-3 hours is the rule of thumb, after which, the expectation of nerve damage or gangrene rises exponentially.
So no, there is no need to worry about any long-lasting damage as a result of using hook grip, unless you plan to hold the bar for several hours straight.