I don't really have a problem with xG if you see it for what it is
A tool to be used as a wider element of analysis.
Just it isn't "unlucky" if your xG is always at odds with your results. It means you have shit players at one or both ends of the pitch.
The xG always being 0.76 actually underlines one of the basic flaws of it as a measure. Because there's a big difference between Raul Jimenez or Nelson Semedo stepping up to take the spot kick, but it's measured the exact same way. Maybe as time moves on they might factor in who's actually taking the shot (this applies to all attempts, not just penalties), it'll be a while though.