@julian I had a very extensive talk about the use of Swap with a friend recently.
As a matter of fact the Swap storage will be always slower than RAM. So what you basically do is to move "fast" applications to a "slow" enviroment.
The best example would be a Redis database, which mainly is so fast because it is working in memory. If the Redis instance would now use the Swap storage it will drastically slow down.
In my opinion it is more than okay to do that in a dedicated system (database only server), but in a shared environment this would mean an extended use of I/O. I/O that is needed by the website itself to serve its files at a proper speed.
Of course the required amount of I/O operations won't be that much for a websites with a few request, but for a mid traffic site things drastically change.
Besides the loss of I/O performance and general slow downs overall, you also increase the risk of storage failure. SSD's might be super robust and performent, but as magnet drives they have "physical" limitations - storage cells.
Using Swap will definitely increase the usage of these and will help to make the SSD die off even faster.
But as said, most of these drawbacks will be noticable as more the site itself grows and a certain level of connections (and therefore "hardware users") is reached.
To get back to the title:
Basically you can run NodeBB on an even smaller server. Thanks to NodeJS the RAM consumption is kept at a quite low level. With the use of a highly efficient webserver like NGINX, MariaDB and HHVM (for your PHP sided stuff). You can definitely run several websites at once on a relatively small setup.
Yet it all depends on the amount of visitors you have and what you "show" them. For example I once saw a MySQL database of WordPress with 60MB of content. A database of this size can definitely slow the overall experience down, if it the system is not capable of handling this. After some optimization the database decreased to 2.5MB (too many plugins...).
Of course the whole database isn't served, but as bigger the server sided files get as more work it becomes for the system.