If Haoushoku was all about making people faint, I guess it would not be such a threat. The top fighters of One Piece can beat thousands almost without moving, so Haoushoku would only be the level up, doing the same instantaneously and without moving at all, and without hurting anyone. Not a decisive power to me.
Haoushoku is likely to have different kinds of powers. Fainting is simply the effect of intimidation on weak ones, and intimidation is likely to affect strong-willed people in a different way even if they don't lose consciousness.
I'm sure an Haoushoku user could make use of it against a very strong opponent.
Let's say that with Kenbunshoku Haki Luffy foresees his opponent's attack, he uses intimidation at the right moment, affecting his opponent's spirit in such a way that he is not in the same condition/mood he was supposed to be when preparing or making the attack. That way Luffy could stop, change or weaken his opponent's attack.
A good example of the people's spirit in a fight would be Luffy vs. Garp when Garp's sudden change of spirit - very likely on his own - changed the whole situation. It was Garp who got punched in the end and not Luffy. Affecting your's opponent spirit - make him doubt for example - could weaken/slow his attacks and change the issue.
If words have a power above other people's mind - and Luffy's has always been expressing his mind to his opponents with words and actions, Oda made clear he doesn't kill them but kill their spirit - Haoushoku could have many powers on people.
If we think about it, words are only the way of one person to communicate his thoughts with another's, Haoushoku could be the power to communicate one person's mind to another's. The others simply listen to the King's mind and can withstand it or not depending on their own will.
And one people's mind can express many other things than intimidation. Haoushoku was never defined as intidimation but the user's very spirit manifest. Since it works with the other people's spirit, I'm sure it can affect people in many ways. It has potentially any power within the range of mind.
Intimidation is simply one kind of expression of one's spirit and each expression would have several levels - intimidation could make people faint, stop themselves, doubt their strenght and convictions, break their fighting spirit, etc... Confidence spirit could boost people's spirit, strenghten their courage and convictions etc...
There may be a lot of expressions and a whole range of effects on people depending on the Haoushoku user's spirit/mood and his opponent's own strenght. King Haki is the power to affect your subject's mind with your own, that's why Rayleigh said it cannot be trained. That's also why it is powerful and useful.