They say that there's no exchange fee now, but they also tell that the conversions are done at "wholesale rate" https://help.wirexapp.com/hc/en-us/articles/207870335-Wirex-fees
What I can tell from my experience is that the conversion in my case (EUR->RON) is done at a rate some 2.2% worse than my old bank's rate. From what I remember, before they had a 3% conversion fee, but I don't know what rate did they use. However, it's a step forward.