I wouldn't call either useless, but which to use depends on the exact scenario. There's basically three options for coughs with support behind them, the problem is most professionals neglect distinguishing when to use which and just prescribe willy-nilly:
For suppression of 'ticklish' coughing (i.e. where any form of very minor irritant in the air will set off a coughing fit): Dextromethorphan, which is an antitussive (and which is psychoactive and depending on genetics WILL make you trip even at therapeutic non-abuse doses)
For 'sticky' coughs where the mucus refuses to depart: Ambroxol, which functions as a Mucolytic, i.e. it makes the mucus/phlegm thinner by causing partial breakdown of them, i.e. it fucks with phlegm composition which in turn fucks with the phlegm's rheological parameters
For 'dry' coughs, where there isn't enough mucus: Guaifenesin, which functions as an expectorant (albeit as explained below, "Secretolytic" seems like the better term by far), ramping up mucus production/bronchial secretions/hydration. (avoid confabulating with Guanfacine.)
And then of course for ticklish coughs/antitussive action, there's two other, much better solutions, but ones with it's own issues: Codeine & Dihydrocodeine. Works wonders — as long as your CYP2D6 doesn't function in an atypical manner, in which case it'll either not work at all or get you way too high.
Part of the problem is that a lot of the literature uses the word "expectorant" in the same way I used it above, but really, the more appropriate word to describe guaifenesin is "Secretolytic".
Obligatory disclaimer:
this comment isn't medical advice, I'm neither a medical professional nor a medical paraprofessional, in case of concerns, contact/consult an appropriate medical professional and/or appropriate medical paraprofessional.
I wish I knew why no combination of Guaifenesin & Ambroxol seems to exist on the market, but there probably exists SOME reason.
To any professionals reading this:
Sorry for the low quality content, I know it's very flawed