The Merc race advantage is the ratio thing that kicks in after a couple laps, or at least its been the theory and more power naturally would mean better options for battery use.
Will be patient until the rule tweaks arrive later in season to judge.
I do think the struggling teams are potentially scapegoating the batteries tbh, and actually haven't managed to maximize their engines yet, may have got some things wrong... well, not as wrong as Honda.... but not yet figured it all out yet. Just look at the Merc customers, they have the same engines, but not remotely the same performance, there like with the introduction of 2014 engines and then the ground effect, not everyone gets it right at first and have to play catchup, just rather easy to blame the battery.
Going to be interesting to follow the season as things develop.