It will make everything you do online a little slower. If your VPN provider isn’t very good it make it a lot slower. It can cause you trouble with services like Netflix that try and determine what market you’re in based on your IP address. If you’re involved in questionable behavior, illegal content or anything else, it makes it appear that you we’re actively hiding your activity which can cause prosecution to be more harsh. The software you install on your device, especially from “free” VPNs can be just another vector to deliver viruses to your PC or devices.
The biggest issue is that VPNs give users a false sense security. People have developed mistrust of their ISPs over the years, not entirely without justification, but for some reason that same level of skepticism hasn’t yet developed for OTT VPN providers, which in my experience are _more_ likely to be a source of problems and (if in the US or a treaty nation) still going to hand over your information when a law enforcement agency sends them a subpoena. VPNs are great tools in our security arsenal, but they don’t protect people from most kinds of attacks and they don’t provide more security to the normal user than their standard connection from an ISP.