If subscriptions are made to work together probably the author puts some effort in avoiding "redundant" filters, that means to avoid to repeat the same filters in each subscription (or even make filters that work against each other). That is the logic of "core" subscriptions and "supplemental" subscriptions.
When you mix different subscriptions from different authors instead you are almost certain to get the above issues, due to different "strategies" followed and overlapping. Fortunately it should not break anything but your filter set as a whole is not "optimized" for performance and it is not automatically "fixed" with updates since each subscription has got its own async flaws and fixes.
About the question "is it useful?", it depends what kind of web sites you visit.
Each subscription is for a specific purpose (covers a small well defined area). The annoyance subscription does not block ads (that is taken care of by other subscriptions). You have to look at what it is designed to block and ask yourself if that sounds like something that you want blocked.
I've tried the annoyance subscription for some time, to me it seems that the sites are shown in the same way as before, but when I looked at the number of hits I saw that it was actually working, so I guess I'll keep using it.
Thanks.