When mishandled, LiIon’s generally give problems in 3 areas: over-discharging (damages the cell), charging damaged cells (BIG problems), and un-monitored series usage. Common sense takes care of the rest. As long as the cells are reasonably matched and you don’t allow any cell to over-discharge, parallel use is almost as safe as single-cell use.
If you’re new to LiIon cells, read this safety primer first.
Phil