Recently, I got a soldering course and we used flux for everything : trough hole, CMS, to solder, to desolder, to make a joint better. At first, we worked only with lead-free. Maybe flux is more useful when we use lead-free and for beginners like me but clearly, it helps a lot.
When we passed to lead, it made a big difference and I understand why people who use it for a long time are reluctant to use lead-free. But the issue is not only for health but also for environment. It is likely it will be completely forbidden, except for some sectors like aeronautics.
So I think to use free-lead before being too “attached” to lead.
However, they told us nothing about the toxicity of the flux fume. If it is toxic, it is not a good idea to use it for everything.