Also, typical lead-free solders oxidize in air at their soldering temperature much more readily than near-eutectic Pb-Sn at its respective soldering temp.
That oxide film tends to interfere pretty badly with wetting of surfaces being soldered. In other words - it's much easier to end up with a "dry" joint - even with adequately increased temperature - unless better and/or more flux is used.
Not doubting the facts you've mentioned, but in my experience I've had no problems getting the surfaces to wet, though that may just be due to using rosin core solder that happens to have a good flux in it.
That oxide film tends to interfere pretty badly with wetting of surfaces being soldered. In other words - it's much easier to end up with a "dry" joint - even with adequately increased temperature - unless better and/or more flux is used.