Well, my 45CL-4Digital unit came with an NiMH battery pack, so their 1.2 volts must be fine. Use the rechargable NiMH batteries and things get better. One thing that has been overlooked in the discussion of voltage differences and that is the category of differences in internal resistance of the different batteries. Perhaps Li batteries will allow too much current, too fast. I don't know what to think of that, though, because my NiMH batteries are fast.
I recently bought a Metz 54-MZ-4i and its manual does not list Li batteries as a permissible power source; it only lists NiMH, NiCd, and Alkaline as acceptable.