Harbaugh and the team parted ways in December after an 8-8 season. Harbaugh spent four seasons with the 49ers, leading the team to three straight NFC Championship game appearances and one Super Bowl.
Harbaugh was then hired as the head coach of Michigan, agreeing to a seven-year contract worth a minimum of $40.1 million.
"He does a great job of giving you that great spark," Boone said to HBO's "Real Sports with Bryant Gumbel. "But after a while, you just want to kick his ass. He just keeps pushing you, and you're like, 'Dude, we got over the mountain. Stop. Let go.'"
"He just kind of wore out his welcome ... He wanted too much. Demanded too much. Expected too much. ... And you'd be like 'This guy might be clinically insane. He's crazy."
Boone also said Harbaugh is a better fit for the college game than pro game.
"I think he's a better college coach than he would be as an NFL coach," he says. "It's different for college kids than pro players. I don't think college kids can say, 'Hey, Jim, you're pissing me off today.'"
Harbaugh admitted that football is "not a fun sport, but what you come to love is the struggle" and that he can be demanding on his team at times.
"It must be true," Harbaugh said. "Because I'd wear out my welcome. People just don't want to be around you for a while."
Harbaugh, who has coached at the University of San Diego and Stanford, was asked if he planned to be coaching Michigan in four years.
"God willing and the creek don't rise," he said, "I'll be right here."
- Scooby Axson