I think, from what I've read from the folks panicking, Theresa May's big mistake was using as a harsh negotiation point -- deporting EU workers/immigrants (many of whom would lose their jobs and be homeless as a result). Whether intended or not. That got a bit of a backlash, because no other manifesto else stated that.

Trump also got backlash on how he's handled immigration -- sentencing many people to death, homelessness, and unemployment. I think the temptation to resolve terrorism with an immigration ban should have been resisted, and again all the factors involved considered. Instead, he just did it, and it bit him in the butt, deservedly so.