Unfortunately I do not have any comments about the FlexAdapter.
But I do own 4x5" field cameras and the mentioned Linhof Techno. In summary, the Techno is a more rigid field camera with a fixed rear standard. All movements are geared and precise, with zero detends. Still there is concern keeping the standards parallel. Exactly this might be the main concern also using the FlexAdapter on a 4x5" camera.
The field cameras I used so far are not in the same league as the Techno when it comes to precision. Either gears are missing, or they are too unprecise. A lot of pro photographers changed from Sinar to a Linhof 679 system for increased precsion for digital use.
On the other hand, others are happy using digital back adapters for their LF cameras (Darr for example). I decided to skip this part as I do enjoy to use my gear for 4x5" film, but am afraid I wouldn't have the same amount of joy when it comes to my digital back.