An IP address alone isn't enough to detect multi-accounting. Factors like screen size, device ID, browser, platform/OS, user agent, and user activity patterns are more commonly used for that purpose. So using a free VPN isn't really a problem in terms of multi-account detection.
The main concern with free VPNs is usually security and a poor browsing experience, as they tend to be slow and unreliable.
Well, I always thought that it could lead to issues or further questioning when withdrawing for example, and require KYC, if you haven't submitted already. Good to know that there are other measures to detect multi-accounting, I knew some of those (didn't think that screen size could also be used), but as I said previously, perhaps it could raise a few red flags, but I could be wrong as well. One thing is certain though, most free VPNs suck in terms of speed, with often disconnects.