to be fair winning second serve points were much harder back then because they serve volleyed on them and the gut strings made it tougher consistency wise.But here's another way to look at it.
The absolute gold standard for 1st serving is Ivanisevic. He won around 87% of his 1st serve points on grass. That's 2% higher than Karlovic. His 2nd serve was very attackable IF someone got to see it. He won around 50.5% of his 2nd serves.
On 2nd serve 59% is the gold standard. That's were Federer is.
The problem for Murray is that his 2nd serve winning % is more or less average for a great player. But because his 1st serve is relatively weak in earning points, he has to serve a lot of 2nd serves, and as you say they are attackable. The reason he wins as many as he does is because of his defensive skills, a lot like Nadal. But Nadal's % on all service points on grass is 1% higher, and Murray's defensive skills on grass are WAY above Nadal's. So he is depending on his overall game to gut out 2nd serve points, and that has killed him. And he is way better on serve on grass than on other surfaces.
If Murray had a better serve he would have a whole bunch of Wimbledons by now.
That's one of many reasons why Lendl is so good for him. Lendl somehow gets him to go for more shots and not to just try to outlast everyone else.
That being said I think a very underrated part of Fed's game is his second serve. I think he has easily one of the 5 best of all time, and maybe even better. Proof is in the pudding as even as his baseline game has steadily eroded his second serve points won have remained near the top of the tour. 2012 and 2014 are particularly impressive as he barely had a top 10 baseline game those years but still his second serve points won led the tour. For example Nadal used to be near the top of the tour in second serve points but when his baseline game went down his second serve won points did too. I think Federer's second serve with the placement and kick just has a great ability to draw a lot of forced errors and weaker replies.
While his first serve is maybe top 15-20 of all time his second serve+clutchness give him one of the 10 best serves imo.