Not trying to turn this into a he's mostly played weak players thing, but has he played anyone that seriously attacked or could threaten him on his 2nd serve?
There's Djokovic, and there's also Santiago Giraldo, who swings for the fences on second serve returns, not to mention Blake and sometimes Gonzalez.
But the issue with your implication is that Nadal's second serve as a single shot is weak, but players now are too stupid / incompetent to take advantage of it, and that is not the case. If you've ever watched court-level footage of someone receiving Nadal's serves, you'll find that his serves--first and second--have incredible action on the ball.
So what I'm saying is that most players can't tee off Nadal's second serves not because they're magnanimous or benevolent, but because they're unable to. Most players can barely handle a barrage of Rafa's topspin FHs, and those come in at 60-80 mph at 3,000 rpm--how exactly are they going to tee off his serves, which have even more RPM and pace that he has almost complete control over?
Just because the serve isn't fast doesn't mean it's a powderpuff serve...okay, they were powderpuff in 2015 and 2016, but then again everything in Nadal's arsenal save for his BH were powderpuff in those years.
And of course, there's Rafa's second shot, which is up there as one of the greatest.