phpbb is dead simple to hack on if you understand css, html, and some rudimentary php.
flarum is hard to hack on and wants you to do things phpbb has baked into the cake via extensions, which requires some serious skills and time to figure out how to write. Even just changing the theme extensively requires writing an extension, which means that a front end designer cannot work on extensive theming.. only simple things like changing colors and removing or modifying elements using CSS.
phpbb is kind of locked into an early 2000's style design and functionality though, leaving you with a very large hack job to move it up to modern standards. It means that you basically need to maintain a fork of the code, or use extensions which are often broken between versions as well.
Flarum's main asset is that it gives you a lot of the modern stuff up front, but customizing it is a lot more work because you gotta work within the extension way. Hacking the code directly and merging it into a new release would be a much huger nightmare than doing so with phpbb.
In theory, you work won't go to waste over time if you go with the extensions route in flarum. But we don't know yet, it's still in beta and the way extensions need to be written have changed over time.
All the nice functionality of the stock flarum install come with the downside of a much more advanced internal structure with many more internal layers and abstractions to sort through. The developers could not suggest a debugger for me to start doing more advanced things and discourages me from doing so, if that tells you anything.
If you have a simple forum that does not require extensive custom modification, flarum is probably the way to go.
I have not found the ideal forum software, so i've decided to write my own because i'm frustrated with everything that is out there.