too frequent tax law changes for foreign income

I was told that income from research and science from universities that is classified as foreign income now is tax free but nobody knows how long this law will actually valid – so I am not moving there. Also, to actually benefit from this law, you need to preregister and get the tax exemption confirmed by the government via a Portuguese accountant, but nobody knows how long the approval process will take. Need more consistency and clarity, tax law sounds like a random thing.