- How much to pay? Since they're paying millions of people, they need to ensure they have enough money set aside for that, or UBI payments will deplete the government's money.
That would vary, if we were to place a central body that would manage the UBI for the entire world, then varying payments would happen due to different inflations of each country, not to mention that each country has different levels of financial assistance they could provide.
- What time interval to pay in - If this is not controlled we get the same consequence as paying too much at a time.
This also depends on how much pay would one get. If a central body were to set the maximum amount of expenses for each group (high class, middle class and low class) then things would probably be a lot easier, but then again, let's not even mention how some people don't have any proper way to regulate their expenses, there's also disparity when it comes to people from each class.
- Should people be filtered out based on financial status (EDIT: and employment status as well)? Presumably finance ministries of governments have records of every citizen's earnings (as in profit not balance), and a UBI proposal needs to be crafted meticulously to be very precise in the criteria for organizing everyone into (only a few) financial wellness categories, a vague set of requirements can be challenged by parliaments and get the proposal blocked.
Financial status should be more than enough, though changes with employee status may change their financial status at times, so finding a solution for that could be pretty helpful. With this issue, it may be a lot easier if they gave out UBI monthly to avoid issues when people get fired from their jobs abruptly.
- Corruption: Some of the budget can be stolen and laundered by people in charge of safekeeping it.
Look at 1MDB for example, $700million was stolen from them with the help of the prime minister.
Nothing we can do about this really. Corruption has and always be somewhere money is.
Let's assume people will be filtered by status, since giving everyone a fixed amount while keeping withing the budget risks not giving the most needy people enough money, the main hurdle faced here is parliamentary consensus of how to classify everyone in different categories so they can set different incomes for each group. And that's complicated because government budgets decrease and sometimes increase so you have politicians arguing that the income should be raised or lowered accordingly and this just entangles entire proposals.
We basically have nobody in power agreeing with each other on how much money to pay each person.
Then how about a fixed amount is obtained from government budget? That, or you could set a sort of tax that varies with wealth which directly goes to UBI funds. Though this makes it look like the rich is supporting the poor, while the poor is making the losses of the rich be at the bare minimum.