Short answer: it should be pretty safe (but I am hardly unbiassed).
There are some things to be concerned about when installing any additional plugin:
- The code may not be the same quality as core Moodle code.
- The developers may, or may not, still actively support it.
- How many people can help you if you get stuck?
So, prudent due diligience might include:
- Work out where it came from, and how committed they still are to the plugin.
- Look to see how many other people use it (how does that compare to other similar plugins?)
- What public comments can you see, and did the mainainers reply?
- When a new version of Moodle is released, how quickly is the plugin updated?
- If you are worried, get a developer to do a code review.
In this case:
- Created by the OU, who still actively use it.
- Usage (https://moodle.org/plugins/stats.php?plugin=qtype_combined) is less than some (e.g. https://moodle.org/plugins/stats.php?plugin=qtype_gapfill) but still quite big.
- Comments at https://moodle.org/plugins/view.php?plugin=qtype_combined&sesskey=nU2bTx2aDN mostly seem to get a reply.
- However, a request in November 2015 for a Moodle-3.0 compatible version did not get properly solved until March 2016. And, right now, there is not a version marked as compatible with Moodle 3.2. (This explanation is still basically accurate: https://learn1.open.ac.uk/mod/oublog/viewpost.php?post=610. There should be a 3.2-compatible version in a month or so.)
- Tim Hunt (developer of the core quiz code in Moodle) has his name associated with this plugin, it is probably safe to assume the code is quite high quality, so you could probably skip getting it reviewed.
As well as the generic points above, you then also have to ask if there are any speicific issues for this plugin. (E.g. https://moodle.org/plugins/qtype_stack requires you to install some extra software called Maxima on your server.) The thing to note about this plugin is that to work well, you also need to install some other OU question types (ou multi-response, pattern-match, variable numeric) are you happy to do that? Since they all come from the same source, there is probably not big issue there.