I think most of us use lead based solder because it’s lower melting temperature and wetting properties is just easier to work with, there’s also easy availability factor and force of habit. No different soldering technique is required(or at least I do not use it) but higher temperature can require more power to work comfortably. With RoHS directive implementation in EU all current LEDs conform to lead free soldering requirements and major flashlight manufacturers should use lead free solder for that market. I am not aware of any problems with lead free solder in LED applications, but I do not reject the possibility. I had no need to reflow LEDs for high power flashlights yet, I did it for single AA lights so I used lead based solder I had on hand. It’s also worth noting that higher required temperature combined with makeshift temperature control (or lack thereof) poses greater risk of LED damage. But then again - pioneers are the ones making history 
There’s also this thread here: Thermal conductivity of solder and pure indium is proposed as a solder because of superior properties (157C melting point, 81.8 W/(mK) thermal conductivity). But it seems it was not put to test ultimately.