It is exactly in between. It avoids saying "yes" or "no." "Arguably, X" often means "I don't think X is true. But I'm not sure. I think there is some evidence against X and some evidence for X." Sometimes, "arguably, X" just means "I am not sure, and I am presenting all the evidence, for and against." For example, consider daylight savings time (horario de verano). I might say, "According to studies, nowadays daylight savings time still saves a tiny amount of energy, but arguably it does more harm than good."

The adverb 'arguably' is neither positive nor negative. It's what we call a hedging device: a word we add when we give a common opinion without wanting to state whether it is definitely true. We often use this for statements which we believe to be true but which have not been verified.

arguable [adj.]/arguably [adv.] means "there is a strong argument (opinion with evidence) possible for this statement"

"Arguably" is the adverb form of "arguable." It means it can be argued; there is reason to believe so, but it's not entirely true or false. If attitudes in America are arguably more relaxed, then there is reason to believe attitudes in America are more relaxed; whether or not American attitudes are more relaxed can be debated. It doesn't not mean true or always.

Thank you