It is commonplace British English to describe an aggressive, violent man as hard. It can also be used jokingly: "you're so hard!" has a sarcastic ring to it. It is possible to conflate this meaning with the sexual one, as Athena suggests: this is what we call 'adolescent humour'.

The common meanings include a difficult person, an unsympathetic person, a violent person. New Oxford American Dictionary: hard • not showing sympathy or affection; strict: my father is no longer the hard man he once was. As an elaboration on Athena's comment, the correct sexual slang in North America for a man having an erection is "He has a hard on." There is also a common women's saying that "a good man is hard to find," and the related sexual play on words "a hard man is good to find."

You wouldn’t say that someone who makes great effort is “hard”. You could however say “He tries hard”. “I admire that ref. He tries hard to be as fair as possible.”

if you want to say he is not soft, but hard, you can say "he is hard". But If you say "he is hard" everyone will think he has an erection. 🍆

It will be more natural to add some more descriptors to the man. If you just say "he is hard" most people will think first of "he has an erection". It would be better to say "He is a hard worker/He works very hard". However the phrase you gave "he makes great efforts" is a much better phrase in my opinion than using the word hard.