As I understand it, you are fine provided all of the batteries used are in good working order and equal states of discharge. It's when a depleted or bunk battery is included with other good cells that problems may arise. I suspect that due to liability issues, manufacturers are quiet about the 123 cells and the cautions must be carefully written.
When KenRad brought out their PSL35 HID, they planned on selling it with both a battery eliminator and a 123 cassette which I believe holds 3 banks of 5 123's in series. When KenRad went to source cells from the battery manufacturers to provide with the light, they were told of the dangers of mixed cells and were warned not to proceed. If I'm not mistaken, I believe the battery manufacturers either refused to sell the batteries or the warning was significant to the extent that KenRad is not offering the 123 cell cassette. There seems to be some real taboos surrounding the use of both lithium and Li-Ion cells in applications where they are placed in series for higher voltage outputs. I have heard enough to have a vague unease or apprehension when it comes to large gangs of lithium cells. At the very least, I have respect for the clustered battery paks and will only use brand new cells when loading them. The Li-Ions scare me more than the 123's but caution is warranted in all cases, I think.
I can understand where a depleted rechargeable cell in a series would essentially be getting charged at a very dangerous overvoltage condition. I don't understand the chemistry to have a feel for what happens when the s$$t hits the fan; especially with the 123 cells.
It would be nice if some real facts were laid out for us to understand.
- Don