That thing can’t have a good FR.
You don't know until you see test results, but it's certainly going to be WAY better than not having any subwoofer in a room at all, because you can't fit one. The 3000 Micro is surprisingly good for such a tiny thing. Heard that one in Vegas at CES earlier this year. SVS 3000 Micro Subwoofer | Dual Active 8-inch Drivers | 800 watt RMS (svsound.com)
I can see the need for in-wall in some cases. However, proper in-room sub placement is tricky, at best. I would think that an in-wall installation for a subwoofer would likely be a sonic compromise unless you brought someone in who really knows how to measure a room's response.