A permanent study would allow you to observe behavioral changes among that group of people, but it won't provide insight into the societal changes. For example, what is UBI's impact on inflation and would it result in inflation that negates some or all the benefit of UBI? Or will a society with UBI still be able to fill that society's less desirable jobs in a financially sustainable way?

It'd still net us more information though. If it works out at least the permanency aspect is answered, and if it doesn't, then maybe we need to reconsider UBI fundamentally.