Tara's situation in the climax of "The Curious Case of the Countless Clues": Steed phones her to warn her that two men are coming to kill her and that he's on his way, expect that Tara's in a wheelchair due to a busted up ankle and realises that her back door is unlocked. Desperate, she climes up her stairs to lock the door, and just in time too.
Being replaced by a Skrull. You can come back years later and everything you know has changed. Your friends and people you have dated might be dead. The Skrulls don't just steal someone's image, they steal their entire lives, sometimes ruining them in the process.
Mockingbird and Spider-Woman's experience are particularly terrifying because, not only was Mockingbird gone for three years, she was hunted by Skrulls after she saw through their tricks, with one of them impersonating Clint while he hunted her. She eventually snapped and killed him. Spider-Woman came back to discover that nobody trusted her after Veranke used her face.