Nice HT room you have !! Are those the Downfiring Rythmik enclosures you have in second pic ??My understanding is that there are no disadvantages, and that a good downfiring sealed sub should integrate into your system as well as a good sub of any other configuration would.
Personal experience tells me that my current downfiring sealed subs blend just as well with my system as did my previous front-firing ported sub.
That being said, sealed subs may "sound" slightly different than ported subs, but that's a difference, not a disadvantage...unless you really like the ported sound.