William's suitability comes into question due to his lack of experience.
I don't think William's experience is an issue at all.
He may not have as much experience as his father or grandmother - but he's in his early 60s, while they're considerably older.
How much experience did Charles have at age 33? Probably more than William does now, although I don't think as much as people seem to claim.
The Queen had considerable experience by the time she was 33, but then she'd been the monarch for 8 years. How much experience did she have at the age of 25? And while I wouldn't be surprised if people questioned her suitability at that point, 63 years later she's proven herself.
I don't think William's lack of experience is an issue at all. I don't think his education is an issue either.
I worry not about that, but about whether or not he wants the role, whether or not his personality is adept at being in the spotlight. Long gone are the days where the monarch can be reclusive. The monarch is expected to be seen by the public, to have a relationship with the press, and to fulfil duties that are ceremonial and full of pomp.
In that regards I worry about William as monarch. He seems to be a person who would rather not be noticed for things than be noticed. He doesn't maintain a good relationship with the press, and doesn't seem to make the distinction between press and paparazzi - and while the paparazzi should be seen as a pest, the press should be seen as a tool that can be used by the BRF or against them. He doesn't seem to take satisfaction from performing the more ceremonial, fluff filled aspects of the role. While I think it's hugely important for royals to have their causes that they champion, the ribbon cuttings and more menial royal duties are something of its bread and butter - and are actually important for the monarch, who is to be seen as politically impartial and serving the people, to fulfill.
What worries me most about William is that in this age, a bad monarch or an indifferent one is one that will threaten the continuation of the monarchy, especially if the monarch doesn't have the press on their side. I don't think William is going to be a bad king, but I worry that he'll be an indifferent one and that he'll raise George to have similar issues.