A major difference between your ISP and a VPN is that your ISP is generally an established company based in the same jurisdiction as you are. So, if they do something terrible, in theory at least, they can be brought to court. A non-trivial number of VPNs that claim to protect your privacy, however, are based all around the world with unclear corporate structures. If they do something terrible, you likely have no recourse at all. How much faith you want to put in a promise made by such a company is up to you - but I would push back on the idea that simply making a promise really provides much value by itself.