When it comes to share hosting it's a whole different game.
It's very strange not to have mod_rewrite on a share hosting but it happens, and like Pete said most CMS/Forums/Social Networks etc... require it to remove the index.php from the URL to make friendly URLs.
In most cases this is necessary, but that's just the starting point when installing an application on a server.
Aside from worrying about a share host having mod_rewrite, you need to see if they even allow an htaccess file to be use on the server. Not only that but then you will have to worry about other requirements that may prevent an application from using all its features.
Some require the GD library and also some share hosts may not enable it or Imagemagick with Imagick.
And the worst of all is mod_security on a share host, because once you have your great app install all of the sudden you can't post something because some characters are not allowed by the share host filters on mod_security to be pass through POST.
So at the end you must know this information from your host not to just install this great application but because any application will have certain requirements and you need to know what your server can handle.
If anyone is thinking of running a medium to large community start thinking about the next level up like a virtual server or dedicated server.
Just because a share host says that everything is unlimited or that they have the latest software is really not true read the fine print and check the extra modules, settings and libraries that they offer because they will limit a lot of things so that your application doesn't use up all the resources.
Remember on a share host you're part of a group of people each one hosting a site on the same server and that is a reason why they limit many things to keep the resources of a server to a minimum.
As far as my experience with Flarum and the installation and use at this early stage is 100% satisfactory to me, one version up and it should be a stable release.
So keep up the great work Flarum team you made a great product and thank you for making it free.