@Mr.:
He probably did use CoA against Blackbeard. Also the fight between him and Akainu was brief. He got burned because he was careless and then he intentionally got hit to save Luffy.
You missed the point. He's saying that we can't generally see haki use, or Oda just hasn't shown it. The few things you mention are the exceptions that prove the rule.
Shanks just blocked Akainu to save Coby. Just saying.
Every time Haki is used there's someone blatantly states that something fishy is going on or We see something unusual. On top of that Haki just got explained step by step what more do you want?