Yes, financing UBI can be a problem. Exact details depend on the exact design and amounts, and what level of taxation (and what kinds of taxes) you use to finance it.
It's not related to financing sources of UBI. If UBI provides „basic“ incomes and you pay „basic“ salary on top, society is now paying 2x „basic“ units.
No matter how you finance it, you can find other (better or worse) means to spend collected money.
The only thing that really matters are net payments (and marginal net payments, ie how many cents can you keep from the next gross dollar you earn). It doesn't really matter whether the effective marginal tax rate is made up of income tax or a phasing out of welfare benefits or a combination of both (and throw in some other taxes etc, too).
In the name of simplicity, you might want to have a single government agency that assesses your income and net worth. Instead of having both the tax people and the welfare agencies do that and duplicate work.
So instead of having welfare payments phase out, you could just increase marginal income taxes by the same amount, and end up with exactly the same net payment structure.
