The ending was completely nonsensical given the established world. Everything in that world is believable, and "magic" is real and based on highly-skilled tricks and illusions. The reveal of the twins, for instance, is a great twist because it is possible and something that you could actually guess / figure out.

The reveal of the teleportation / cloning machine is just flat out retarded and very much a deux ex machina. There was no way to figure out that twist because it is literally impossible. Here we are in the 21st century and that tech is not feasible, and yet somehow it features in an otherwise realistic 18th century period piece. Retarded.

Nope, completely disagree. The cloning machine at the end fits in with the theme of how a magic trick works. Listen to how the "three parts of a trick", and specifically the prestige, is described:

Every great magic trick consists of three parts or acts. The first part is called "The Pledge". The magician shows you something ordinary: a deck of cards, a bird or a man. He shows you this object. Perhaps he asks you to inspect it to see if it is indeed real, unaltered, normal. But of course... it probably isn't. The second act is called "The Turn". The magician takes the ordinary something and makes it do something extraordinary. Now you're looking for the secret... but you won't find it, because of course you're not really looking.

The movie spends a lot of time setting up the idea that every magic trick has a mundane explanation, that there is no "real" magic. It has you so thoroughly convinced of that, that by the time Hugh Jackman's character shows his final trick, the audience is trying to figure out how he did it. Doing that trick without actual magic or doubles would be incredibly complicated. The real answer is actually very simple, just not the one you are looking for. The turn wasn't deux ex, it was planned from the beginning.

Also: it's completely irrelevant that the technology isn't feasible. It's a movie. I don't think the audience was supposed to be able to figure out the twist anyway.