Rin is hopeless because she's not interested in technology, coupled with a good deal of plain clumsiness. By the time she has children, Rin has become comfortable enough with technology that she's generally alright with basic appliances, though she still complains about smartphones and the like even while being envious about their incredible versatility. She still makes mistakes on occasion due to being clumsy, which usually ends in someone bullying her.
Ritsuka, meanwhile, is proficient with technology, and very much enjoys having a smartphone. He's still his mother's son, though, and a combination of having-other-priorities and plain bad luck ensures that he never keeps the same phone for longer than... say, a year.