Amalgamations are a tricky one. Personally, I don't have as much of an issue if it's something like Green Lantern or Dr. Fate; when the powers are tied to an object, who's to say that a Marvel character wouldn't find it before a DC character or vice versa? But I always try to keep it in the realm of "which characters run in the same circles?" Carol Danvers is a perfect Lantern, IMO, because she's already so tied to space. Many of her allies and enemies transfer over. But something like Matt Murdock GL? It's trickier.
I can see the potential with Daredevil and GL based on the nature of his powers mixed with it's power. Part of what makes Kyle Rayner a great GL is because he has a grand visual mind, leading to amazing constructs. How does a blind man take that and make it a strength, especially when his senses largely rely on nothing that exists in the vacuum of space? It's a story that feels worth telling, despite the contrast.
But let's take another amalgamation: Batman and Iron Man. Both are rich orphans who use technology to fight crime. One is a deadly serious obsessive, the other is a good natured narcissist. And combined, it's... boring. You either get Batman in an iron suit or Iron Man with a bat motif and little else to really work with. Maybe a few fun rogues gallery mash-ups, but with Batman's trouncing Iron Man's, that just seems dumb after a certain point.
My point is, you've got to think of it thematically, too. You've got to value what stories can actually be told and be excited for them, otherwise it's just a gimmick.