How to build a 18650 battery pack without a spot welder?
Sure there are downsides to soldering, but it is not like you have a choice, unless you want to spend $$$ to get a spot welder. If soldering directly to the cell, use flux and a BIG (>4mm, even 6mm or 8mm) short, fat iron tip with lots of contact area and be fast to get in and get out. Tabs are safer and easier but less compact.