Some penal battalions did exist in the more manpower strapped countries like Germany (called Strafbattalions) especially after the war situation went poorly for them. They were typically sent on more dangerous/suicide missions since they were seen as somewhat expendable. I believe the Soviets may have used some as well in a similar capacity during particulary desperate times. Don't think the UK or France had them, the US sort-of did by allowing Japanese-Americans who could at the time be effectively called prisoners to fight (442nd Regiment among others) but I not sure if you could really call them a "penal unit". But overall prisoners and criminals were usually used for labor rather than battle as they were typically more useful and reliable that way.