You do not have to pay tax or social security contributions; your gross salary is equal to your net salary.
You do not run the risk of falling into a higher tax bracket because you do not have to include the income from your flexi- job in your tax return (up to 12.000 euros per year).
You build up social rights, such as unemployment benefit, pension, holiday entitlement, etc.
There is no set maximum number of working hours as a flexi, but the government indicates that the wage ceiling is equivalent to 880 hours of flexible work.