stacking or series can always increase the issues and problems, IF each of the cells are protected it shouldnt cause any safety problems, because the protection will cut out as it reaches the areas it shouldnt be in.
"As it reaches", meaning the protection is the last ditch fix, it is preferable that the light or the user not reach the protection levels, even though protection can keep the battery range much safer even when used in series.
when making series Packs or using unprotected cells the application of pack protection would accomplish the similar tasks to the battery protection that is on single cells.
Without protection of some sort, series would be one UnSafe thing you could do with li-ion, because you can "reverse charge", overcharge, or damage one of the cells then when you do charge it will be unsafe.
so Never will series be "as safe as" single, by default, With protection it will be safe enough to use , series or single. the protection is the key item for li-ion.
if your worried about stacking burning out your LEDs or Drivers, meaning unsafe for the components , that would be a different discussion.