the modern stuff works better than leaded with the correct choice of flux-core & iron temperature.
I don't think you'll find many credible sources which agree with that claim. "Mission critical" electronics tend to be exempt from RoHS and similar regulations, such as military, aerospace, and medical equipment, because it is inferior in nearly all respects to the ages-old industry-standard eutectic tin/lead alloy (63/37). The PCB factory I used to work at started using lead-free solder for some things about 10 years ago (which was after I left, but according to a friend who still works there, the employees hate the stuff), presumably for tax incentive purposes. But the line I used to work on makes PCBs for elaborate, commercial-grade fire alarm systems, and they still use normal 63/37 solder, because fire alarms are considered "life-saving equipment".
On top of the joints being inferior in most respects (and subject to developing
"tin whiskers"), the increased heat that SAC solder calls for is harder on components and pads as well.
Lead-free electronics solder is only "modern" due to modern politics/bureaucracy. It isn't a technical advancement, it is a step backwards. Tin/lead solder is what replaced pure tin or nearly pure tin solder ages ago, mainly in order to solve the "tin whiskers" problem.