Please note that among bow instruments there's also the hurdy-gurdy, which is very traditional too and does also employ a "rotating bow of even tension with constant force". One may like its sound or not, but for some thousand years there seem to have been quite many people who did like it. Thus I believe it renders big part of your argument moot. That said, the hurdy-gurdy allows for variable speed of the wheel, so some more variation in this area, but I don't believe it's right to say the viola organista is bad because it is different than violin.
Furthermore, because of its "trumpet" and bourdon strings, the hurdy-gurdy also allowed a bunch of extra sound effects. It has been called the "synthesizer of the middle ages".
The hurdy-gurdy also allows some variation by using more or less force when pressing the keys. This may also be the case with the viola organista, although I suspect less so.
