For an aspiring early retiree who is maxing tax advantaged space and intends to live off of a Roth conversion ladder in the earliest stages of early retirement, rental property income has some drawbacks.
Rental property income is taxed as ordinary income (less FICA). You do get to tame the number down via expenses, depreciation, and the mortgage interest deduction, but whatever is left will fill your tax free space first, just like a pension. For example, I could rent my house for $24,000 per year. After operating expenses and depreciation, I'm left with $8,000 in taxable income. Which is cool and all.
My family of 4 has tax free space up to $28,200, and I do intend to receive self employment income in ER which, considering the double FICA issue, makes my concerns with rental property pretty irrelevant now that I have it all typed out. :)
I suppose the consideration to make is that if you have multiple rentals, you'll always be filling up your lowest tax space with that income and your only real option to maximize lowest tax rollovers would be to sell the properties to get rid of that "ordinary income." If you're working some to supplement your retirement income then you'll have more flexibility to pick and choose your rollovers in low income years.