RDP'ing to your server is about as safe as SSH'ing to your server in that those particular channels are encrypted, but there is a possibility that your system can be poked and prodded through other means if it's a fake network made to capture traffic or take advantage of exploits. You should make sure it's firewalled and extra services are turned off; Windows especially seems to have a lot of automatic features made to make things easier and/or more convenient and can potentially cause problems.
VPN's would encrypt your traffic better if you can configure it to tunnel all your traffic, not just traffic to a particular route.
So in general yes it's safe enough for general use (practically speaking) but you're still open to other attacks and exploits by using an open wifi. Keep your system up to date, run AV software and malware detection and keep an eye on your logs.