This is my grandmother's Frigidaire microwave. She has trouble setting it because she finds the button layout confusing. Or at least she did when she first got it - she's remarkably adaptable.
One problem is if she wanted, say, "1 minute 30 seconds", she'd push the 1 but if she didn't get on to the next number (3) quickly enough, the microwave was like, "Oh, easy set!" and went on for one minute.
Another problem is that it's not clear at a glance whether one should enter the power level first or the cooking time first.
My grandmother's previous microwave had a dial to set the cooking time. She much preferred it. Naturally, it broke, and there were no more microwaves with dials available when she needed to get a new one. If I were designing a microwave right now, I'd centre it around a dial but also include a LED and a digital power level selector that defaults to "HIGH / 100%" so that the complexity is there if you need it but otherwise it's entirely optional.