No, there's no advantage. The players ERA is a ratio to an adjusted (for park) AL or NL average.
When that league is a stronger offensive league, that player can have a higer ERA to score 1.10 ratio. When that league is a weaker offensive league, that player must have a lower ERA to score a 1.10 ratio. It's that simple.
The flaw is that with the unbalanced schedule, it doesn't account for divisional strength within the league. A pitcher in the AL East will have his ERA+ computed to the same AL average as a pitcher in the AL West, which isn't really fair. Yes, it's imperfect, but it's not an ineffective way of accounting for the differences in leagues like you suggest.