Note that I am a fan of Danny's servo subs both sealed and OB; I've heard the OBs often enough, and have owned sealed box versions more than once, only selling to downsize, never to replace or upgrade.
That said, some possible compromises for the OB:
Limited off-axis output. This is a good thing for managing the room to improve quality of sound when critically listening, but might not work out so well for you if most of your listening is more casual. If you need very low frequency output while walking around the room/house/condo, a box of some kind might be the better way to go. You could always add more drivers to increase total output, but for many this is a solution with a finite practical limit. This also goes for filling a larger dedicated room with OB sound. Danny's OB servo drivers are purpose-built overachievers in this regard, but still something to keep in mind.
Space/positioning requirements. Like anything OB, these really should be pulled out a ways in to the room - three feet at a minimum, usually anecdotally more seems to equate to better. You can get away with less, but with some compromises in the sound quality characteristics you're chasing after specifically by going OB. Caution must be taken to keep in mind that for an OB speaker, the room becomes the box in which it must play nice, and the more open air you can get to get the speaker as far away from that "box" as possible, the better your results. There is a little more leeway to be had with the side walls, but I have heard a noticeable improvement even there with additional space. Domestic tranquility or practical limitations of shared-room environments might prohibit usage where a box against a wall may not. Also, frustratingly, OB subs solve a lot of challenges in very small rooms, but again due to positioning needs, there might not be enough floor space available to put them where they need to be to work if you don't want or can't have a possibly fairly near field setup with a single listening chair.
Intended usage. Many people have their systems fill duties both for music and movies. If you are an action or effects movie kind of person, as discussed earlier, it is easier and cheaper to get THX/Dolby reference SPL levels (often times specifically intended to pressurize a (also to be fair, to-spec purpose built, likely overbuilt bunker-like) room in ways explicitly counter to what the OB design is meant to do using boxed subs of some kind. For this, a swarmed sub approach or an IB Ed mentioned might be preferred even to just a single, big, high quality sub to deliver quality bass to as many seats in the room as you can, accepting that you are compromising getting the absolute "best" bass at one seat in the exchange. I can say from experience, though, even if you enjoy that kind of movie, if you don't listen at reference volume levels and are not in an overly big room, the servo subs in boxes pick up details many other solutions miss while still delivering bass you can feel.
Aside, speaking from the perspective of a music-only system, some folks prefer their bass to kick them in the chest more than they value very low articulation (or don't listen to anything needing output below 30Hz or so), others look for detail first and shaking their ribcage is a far second (even if that detail needs to be delivered for an 18Hz note to get all of what is in the music), depending on tastes and music preferences. One path may be easier to achieve one's primary goals than another. Danny's example of a drum kit outdoors is a good one, but if your goal is to recreate the experience of an amplified band playing in an enclosed small venue through stacks of on stage or in the club bass speakers, a boxed sub solution might actually get you closer to your intended original source than an OB. I say that knowing full well that that flies in the face of the priorities and goals of a broad swath of the average audiophile I have met, and although it may not even be my personal priority on any given day, it is no less valid when someone is picking out equipment to make a music system that makes that person personally happy.
So, I guess from my experience and understanding, I don't think it is the quality of sound or advantages OB subs can provide that are taken to task in comparing to sealed or ported box solutions, but whether they are on the table as an option for you given the compromises they call for in implementation, and how closely their compromises align with your intended usage. I think the discussion starts with picking the best plausible tool for the job, or another way, determine what you CAN'T use due to unacceptable compromises, then look at the choices you have among what is left.
