If you are employed to work 9-5, you work those hours (and possibly outside those hours). If you want to work 9.30-4.30, or 10-6 or whatever permutation of that, it's a flexible working request. Not a oh I'll just start at 10 3 days a week if I feel like it.
Many years ago I did flexitime. We had core hours of 10-4 of when you had to be in the office (albeit you could take up to 2 hours as a lunch break). You could only accrue flexitime after 7.30am or up to 7pm. Thereby stopping anyone working 12 hour days or starting/ finishing excessively early. It worked pretty well and was never abused.
Would it work with people working from home? I highly doubt it.
In my department our outbound call volumes have fallen off a cliff in the last 20 months. They are down by 75%. This is because there are a lot of staff slacking in work hours, getting the bare min done but outside of 9-5, I've heard examples of people logging on at 6am, sending a load of emails then basically logging off at 8 to deal with kids and so on. The problem with this is that the emails they're sending are not getting the job done. They need to get on the phone to do that, which can only be done during the hours that other businesses are open or 9-5.
My DP has his own business with several people working for him. He's in the Tech sector, absolutely not a 9-5er, but works whatever time he needs to in order to get the job done and expects the same from his employees. He absolutely has encouraged them to set their own hours, has a duvet day policy, is happy for them to say they are just not in a work frame of mind on a Friday and not work but put in some time over the weekend, that kind of thing. All pretty reasonable and he pays well.
And yet everyone working for him is a complete slacker. If ever a Teams call goes on past 5 even though none of them start at 9 they all start rolling their eyes/ disengaging. Requesting someone to complete a task between Friday and Monday (when it's already a week late) is ignored. It's rare for any of them to work anything close to 8 hours a day (logging on at 10-11 and off at 4 at best is commonplace). Productivity is way below where it should be. As an employer DP could not have been more accomodating - yet that flexibility only works one way.
It's nice to think that flexible, helpful employer = staff that don't take the piss. Sadly that's not always the case.