In addition to not building up a pension as a freelancer, it's important to note that you also don't build up vacation pay. 

If you work in salaried employment, you will receive annual vacation pay. Often this is at least 8% of your gross salary. Because as a freelancer you do not work as an employee, you do not automatically receive vacation pay. You can, of course, pay this out to yourself. For example, you can set aside 8% of your earned money each month as a freelancer, so you still have a nice savings pot of vacation money.