Any realistic UBI scheme works by adjusting marginal tax rates to compensate. Everyone gets the BI but it’s effectively taxed away from those with enough other income. Assuming the BI replaces many of the current benefits, it can be implemented by only slightly increasing tax rates for the highest income brackets.
Finland currently has unemployment at 5.4% and working age economic inactivity (i.e. people who do not pay income tax but are also mostly not eligible for benefits) is closer to 30%.
If the state is paying out the equivalent of a livable income to >5x as many working age people as before that's a big increase in [net] income tax burden on the employed. Given that many of these economically inactive people are not looking for work by choice (probably because they don't need the money), there's a big question over whether that's an income tax burden worth imposing.
It is true that the best place to fund UBI is not from a pure income tax. In the same way raising the minimum wage inflates the general cost of goods, but at a lesser rate than the poorest earners see their incomes rise, having a UBI with a (preferably progressive with annual per-capita credit) generalized transnational / sales / exchange tax across the whole economy would work better. That would include income, but it wouldn't exclude capital gains, trade, consumer purchase, etc.
