Amnesia really sucks as a plot device. Occasionally, it can create interesting, although not dazzling, dilemmas. IZombie's current "the zombie cure might give you amnesia" plotline isn't the best thing it's ever done, but as it affects one of the villains and not the protagonist, it does create new dilemmas in the story without sending it off the rails.
But when the protagonist of the story gets amnesia, and especially when the amnesia crops up in a romantic plotline... then, things get a little more frustrating.
Take, for example, the recent fantasy novel, The Winner's Kiss. Don't let the title fool you. This is the third in the Winner's Curse trilogy, where romance sits on the back-burner for political intrigue, moral dilemmas and great writing and character development. To say I was looking forward to this book would be an understatement. The protagonist, Kestrel, faced some serious consequences for her scheming at the end of the second book, and I couldn't wait to find out where her story would go next.
But a few chapters in, she gets her dramatic new plot twist. She develops amnesia, forgetting pretty much every plot point in the previous books, and every character she met. She spends the rest of the book referring to pre-amnesia Kestrel as a different person, and it genuinely feels as though our protagonist has been replaced by an alternative universe version, thrown into the last third of the story without any context at all.
Despite their soap-opera connotations, you could argue that there is a literary purpose to amnesia plotlines, as they allow the author to deconstruct a character by taking away their memories and exploring what remains. But I don't read the third books in trilogies for abstract character deconstruction. I want to see the characters I've grown to love deal with all the drama and danger that's developed over the past two books. And amnesia plotlines rob readers of that satisfaction.
Whether deliberately or not, amnesia stalls the narrative, or sends it flying onto a separate track altogether. In a story like The Winner's Kiss, it's a cheap way to keep the Main Couple apart for another 2/3rds of the book when the story has run out of natural obstacles. He loves her, but she can't remember that she loves him! She lied to him before she lost her memory, so he doesn't know how she really feels! They have to build their relationship again from the beginning, but with the spectre of that lost past between them. It's an angst device that makes all of the protagonist's previous feelings irrelevant. If the character eventually gets their memory back, it feels like an empty way to pause things for increased romantic tension with no long-term consequences. If they don't get their memory back, it's like a do-over -- either to fix the fact that they were written into a narrative corner, or because, again, they've run out of obstacles to keep them apart.
At worst, it robs the character of their whole narrative, as they can't remember what they were pursuing or why they were pursuing it. Without these memories and motivations, the character transforms into a new one, and readers are forced to continue the story with them without any hope of true resolution to what came before.
And amnesia plots almost always have an element of randomness to them, of something that happens to the protagonist, rather than something that feels like a natural progression of the character's plot. Even in The Winner's Kiss, where Kestrel loses her memory because she ends up in a work camp through her own actions, and then gives in the drugs they give out, the amnesia never feels like a natural part of the story after she escapes. The world around her has been changed by the events of the past couple of books, but Kestrel herself is thrown onto another narrative track, and far from being the passionate heart of the story, she becomes distant from it. It's not her world, these aren't the consequences of her actions, and she spends most of the time scrambling to understand what's going on and figure out her place, rather than driving the story on.
And reading about a character learning things that we've seen them learn before doesn't make for a particularly gripping story. It's not necessarily a feminist issue, unless amnesia is used to give Jerk Guy another chance at love, but it is a frustrating narrative issue. There are better stories to be told.