Re: Users abandon the site after they login, seeing WordPress backend
There are workarounds for this, but the real issue is that this is the intended outcome for normal WP. Since it is the intended result for WP, it gets pulled into WPMU since it is synced off the WP development. Since BP works on WPMU, it gets the same result. See the pattern 
That “block admin” plugin works well and if users login from the main site, it doesn’t direct them. Only when they hit wp-login.php will they get redirected to the dashboard. If you are a WPMUDEV.org premium subscriber, they have a plugin that changes this default behavior, so I would imagine there are other solutions out there as well.
