Who are the "they" you're talking about? Do you know something that UL doesn't or have you fallen for the scare tactics on one of the web pages that I swear is funded by the makers of the copalum splices?
Personally Tom,
There is absolutely no way that I would EVER put an aluminium wire and a copper wire in the same termination and call it "safe".
Sure the copper wire might be fine, but screwing a wirenut over an aluminium wire, is just asking for trouble.
These sorts of connections depend on friction only, not the sort of thing you want to subject aluminium to, as it has the habit of "flowing" under mechanical stress.
Especially when these connections are also carrying current, which will heat any sub-standard connection up, possibly to the melting point of the metal.
Bi-metallic lugs and splices are there to be used for a reason, otherwise, we wouldn't have them.
