The antonyms of "were deficient in" would be "had an abundance of", "were proficient in", "were competent in", "excelled in", "were capable of", "were adept at", "had a surplus of", "had plenty of", "were abundant in", "were rich in", "were bountiful in", "were chock-full of", and "were brimming with". These antonyms imply that the subject had no shortage or inadequacy in the thing or skill referred to. Instead, they suggest a plentiful or overflowing amount or a high level of proficiency in the area. Using antonyms for "were deficient in" can help to create a more positive and confident message in your writing or speech.