For the people who like the strategy, I think the answer is "both / and." You do need to build into your budget some carrying costs during the rehab period, before you get it rented. During that point, you won't be earning income. But after it's rented, it's both an income and accumulation strategy.
My example is a somewhat modified BRRRR, but it might be illustrative: I bought a threeplex that only had two units rented and needed a lot of work. The "rehab" included a bunch of stuff including new roof, getting the third unrented unit ready, and improving an additional space to make way for a fourth unit. During the first months, it earned me no income. That said, the rent from the two tenants did help defray the carrying costs. Now, the property is fully rented (still going to do some more work), and earning income.
I have just begun the process of refinancing. I intend to use the new/reclaimed capital to do another investment. After refinancing, the current (now) fourplex will (still) provide some income. And the equity will also (knock wood) be a tool for acquiring/accumulating property.
(My modified version: 1) Because it wasn't vacant, the rent helped fund the carrying costs. My Rehab was slower. This might have costs to the speed of the next property, but 2) it meant that I got the benefit of the property manager's experience to inform the rehab. Together, we served as the general contractor.)
Hope that helps a bit.