I see all these LV system are installed using what I call lamp cord; stranded multiconductor cables. I have a back yard system that was wired using solid 120V 3 c Romex in conduit to each incandescent 120V floodlight location (it all no longer works well---). I've checked the cables and found continuety from source to each box location, so the cables are ok.
Since I also have a Big LV FX brand transformer for some other more recent lighting, can I just wire up the Romex to the FX transformer and use some new 12V lights on the existing cable?
Is there any advantage to using the multistranded cables? I would have to pull out the Romex and pull in the 12/2 ms-which might prove difficult.

 

Using the existing cable will be fine for low voltage.
Just for a note: Indoor Romex can no longer be used in conduit that is installed outdoors.

 

First, if the cable you have installed is actually Type NM or NM-B cable it is the wrong cable for that environment and should be replaced. Continuity testing does not check for the integrity of the insulation.

Secondly, many LV cables are #10 to reduce voltage drop.

 

Low voltage wiring is very safe with no risk of injury if something fails or has failed with the cable. If something did fail at best is would not work, at worst it would wreck your transformer.

IMO your ok with using the cable for low voltage, but not for 120v anymore. As PCboss pointed low voltage is affected by voltage drop. You many want to replace the cable in the conduit with some #10 THWN.