If you overdo it and really hurt someone, there may be criminal charges filed in some jurisdictions.
And if you have to testify under oath that you have had special training for self defense, the right kind of lawyer can talk a jury into bankrupting you.
Still, the skills are very useful to have.
Take the Fifth Amendment protection; that's what it's for.
One fellow I know was jumped by four frat boys, one of which was holding his arms while the others punched him. Leaning back and administering a roundhouse kick to the head put one of the frat boys on the sidewalk, snoring before he hit the ground, and ultimately in the hospital, and lackadaisical police investigation and anonymous witnesses combined into a criminal prosecution of the victim of the attack, leading to a suspended sentence.