Reflashing is fairly straightforward, so it's not dangerous (except to the applications/data installed on your phone) - but this is something you *should* be keeping in mind whenever you use something in extras-devel anyway: http://wiki.maemo.org/Extras-devel

Now, bear in mind, this isn't a likely outcome in your scenario, but I never recommend something that has even a chance of backfiring without adding a disclaimer.