* Their management interface is garbage, especially compared to sites like Linode. Pay your bills, restart the server, cpu graphs, set up rDNS and that's about it.
* Customer support response times tend to be ~1day or more.
* The network has been down for a period of over an hour twice this year.
However, for the value for money in terms of hardware, I don't mind overlooking the above, especially considering it's just a game server.
For anything more important, you might want to consider other providers, though I'm sure customer support is better on the higher end packages which come with phone support and so on.
Vanilla has had 14 people without issue. It's CPU usage is minimal compared to the FTB pack. Could probably go to 20 or 30 with no issue.
But there is a option called "netboot" which looks like it might be what you are looking for, but unfortunately I can't try it now on any of my servers.
Not as efficient as a direct console access, but for the occasional "Oh , I just firewalled myself" moment, it's enough.