Is Dubai really 100% tax-free for property, or are there hidden costs?

Not entirely. There is no annual property tax, no personal income tax and no capital gains tax for individual owners. But 'tax-free' means no recurring taxes, not no costs: expect a one-time DLD transfer fee (~4% of price), registration and admin charges, 5% VAT on agent commission, and, for occupied homes, a 5%-of-rent housing fee. Confirm current fees before budgeting; this isn't tax advice.