With structured PTO, there is a set number of days employees are given to use. Because you have been explicitly allotted this certain number of days, there is less scrutiny/judgement for taking time off so long as you have a positive balance of days. Alternatively, if you don’t use them, they can often be converted into cash or rolled over.
With unlimited PTO, there becomes increased judgement to take time off since there is no specific allotment of days. It creates additional uncertainty and takes leverage/ownership away from the employee. Every request to take time off has to be done with a bit of groveling/somber attitude and you may even find the need to provide a specific reason depending on the boss/office environment. There is also no option to convert the PTO into cash. Taking off time is seen as letting the team down/avoiding work/lack of commitment since there is no specific allotment of days that are “your’s”.
I would rather have 7 days of structured PTO days instead of an unlimited PTO plan. It is that bad.