>>58025536mega sableye's fall from grace in gen 7 is a testament to balance megas being better than defensive ones. magic bounce is great, but it's still too passive in gen 7. in gen 6, it definitely had more offensive pressure due to the lack of strong fairy types barring mega gardevoir who isn't the best thing to fit on a team.
venusaur is not an offensive mega by any means, but it's an underrated mon in gen 7 that is very hard to break while it constantly spreads status through sludge bombs. altaria and scizor have also taken more balanced roles over time.