Pretty much. From what I gather, many of no-clean fluxes used in industry were barely active enough to do the job. So when lead-free came along requiring another 50 deg. F or so of temperature and a slightly longer heating time, the weak fluxes boiled off before the joint was complete. So when you see “lead-free” on flux or flux-cored solder, it’s generally a little bit more aggressive and designed to activate at a higher temp. Some of them definitely smell worse.
[OKi pdf about the increased risk of lead-free fumes. It’s an ad pushing their fume extraction system, but I think the basic facts are right.](http://www.okinternational.com/binary/articles/Lead-free_Fume_Extraction.pdf)