The red/green was an underrated design and color combo in it's day and too many "purists" hated it. Today, they'd probably be more accepted especially as the league's color pallet has expanded. Personally, I think a lot of the dislike came from the fact the Devils were a bad to average-ish team for the majority of the time they wore it.
FWIW...I can not stand when today's fans refer to them as "Christmas tree" jerseys. It's incredibly annoying. For example, no one calls the Flyer's jerseys "Halloween" or the Wild "Christmas Tree" as a common moniker.