I'm a php developer of 10 years, and i also run two forums.. one very large and spanning 11 years, and another spanning 3 using phpbb.
I really like flarum and would not bother providing any feedback or hanging out here if i didn't like it. This is the only forum project i am paying attention to, other than vanilla forums.
I just think it is not wise to do these things:
1) Use beta software and be surprised when things break.
2) Develop software and provide excellent support to users using a beta and be afraid to break things that make the final design work excellently.
Phpbb is a good example of a lot of bad or just wildly outdated ideas carried forward throughout time, as a means of continuing to support users at the expense of being modern.
Phpbb's failure to modernize is a huge black mark on the entire internet and is the prime reason why @#$@#% like zuckerberg own around half of internet discussions these days. So, projects like this one have the burden of getting right what phpbb and other lookalikes got wrong.
So go ahead and build your forum on beta software if you don't see any other options and be a flarum pioneer. Just acknowledge that there will AND SHOULD be a rocky transition to version 1.0.
The positive is that if you provide good feedback at the beta stage, you get a better platform later.
Not an easy choice to make at all!
The third option is to financially support the project and wait and see how it goes, which is the route i am taking.