It may, however, it put your application in an "unstable" state, where users are sometimes created and sometimes not, so that you need to "store" every configuration and rerun it every time :
In the case you want to create OTG (on the go) the users, i'd put it in the WP plugin side, because that's where the (potentially new) user lands.
One of those scenario is when you want to ban an user from accessing the blog, you may ban an user that doesn't even exists yet, which is weird.
Another scenario is Wordpress plugin : by using the full synchro, you can use the wordpress plugin (mailing to mailchimp for example) which extend your capacities of connectivity of your userbase
In the end, it requieres double the effort because you need to duplicate all the workflow of synchronization from the phpbb side to the wordpress side, whereas my goal was to avoid this now that we have events to which we can listen to (which is smarter in the long run)
The only drawback is see so far is the long, long initial setup for users, in which i need to work on to disply something while it's working in the background
Eventually, it may have an impact on WP performance?
Feel free to counter-argumentate, I don't bite