Complex sentence: They were very successful tank destroyers but performed poorly when used out of role as substitutes for tanks or assault guns, such as in the later stages of the war, because there was often nothing else available.
Split into simpler sentences: They were very successful tank destroyers but performed poorly when used out of role as substitutes for tanks or assault guns. In the later stages of the war however, they were increasingly used as tank substitutes, because there was often nothing else available.