I'm a lefty. Played in 12-14 A/Open tourneys in the SanFrancisco area back in the late '70's.
Every lefty is different, just like righties. Some hit consistent strong forehands, other's don't. There is no general rule except to probe the backhand first and foremost, then if that isn't working, probe the forehand.
Most lefties seem to be athletic all courters, not all. So they tend to get bored quickly if you hit high deep balls up the middle. Me, of course.
You know where their backhands are, so serve there if you want a weaker, more consistent return.
Federer hits a fine inside out forehand vs Nadal and Meltzer, but they get there oftentimes.
Watch your backhand on their serves, especially the ad court wide serve, either first or second.
Good defense against speed and athleticism? Hit behind them.