As advised in the latest beta.8 release discussion, Flarum is beta software still. You shouldn't be using it in production unless you know what you're doing and are prepared to tackle any issues that may arise at any point in time with future releases, extensions, etc. One thing I will say though is that the more extensions you have then the more likely your Flarum forum could encounter an issue at some stage. I have been using Flarum for some time now and whilst I don't have a busy forum, I don't run any extensions and therefore upgrading/updating has been flawless thus far or with minor headaches.
Some people are lucky and never experience issues however others are not so much.
A quick search of this forum will produce some common issues people are experiencing especially with the latest beta.8 release. The majority (if not, all) of extensions have had slight issues as beta.8 introduces a whole new API. The nitty gritty details is better left for someone with greater knowledge than myself but you get the picture. Have a read through the discussion I linked to and at some of the latest discussions on the homepage.