I don't know, without testing, whether there are issues with non-ipsec enabled hosts communicating with ipsec enabled hosts.
On the whole subject in general.... We (HAMS) have been doing Internet connections on Ham frequencies for over 20 years. Every time something new comes up or a new group wants to do it (HamWan...) these same discussions are held. Of course with the rate that technology is advancing the descussions are more involved and sophisticated every iteration. Bottom line from the high water mark - there's never been an issue or a problem. Of course today there is far more speed available to do cool (and sometimes questionable) things. Which comes back to a even more basic issue. Why bother with Part 97 (HAM) use at all unless using frequencies that are not available to Part 15? Part 97 only brings issues, discussions and drawbacks... 73 Bill - WA7NWP