Is it ethical for authors to reference another paper, but not cite it formally, because they consider it unscientific?
As editor I would not accept this in a publication. If it is published it should be referenced. Yes, it bumps the references for the authors and yes, bad science may attract a fair amount of citations for all the right?/wrong? reasons. But, it is not up to the authors to decide how referencing should be made, journals have guidelines that should be followed. Having the citation properly referenced makes it easier for others to find the article and see it for themselves. 

Furthermore, from another point of view the statement that something is "unscientific" is not appropriate either. It is an opinion. The cited paper can be unscientific but the academic way to show this is not to just say it but to prove it.

Your quote is a specific case, of which I know nothing, so the reply concerns the general case but I would react if I saw something like that in a paper I edit and I would ask the authors to stick to facts.

One last point is that if a paper is really bad, then it should be considered for retraction. That is how scientifically extremely poor, bordering on dangerous, papers are handled.