I think location, location, location such a big deal because it helps primarily to identify 2 of the tangible numbers in a typical analysis:
1. Vacancy. If the properties are in "bad areas" you probably will have less interest in the units when they need to be rented, and attract not so ideal tenants that will give you headaches (late/non-payment of rent, don't take care of property, etc.)
2. Operating Expenses. Cheaper properties usually have more that needs fixing. They also might be in areas with higher crime rates so home and car break ins, property damage like graffiti, etc. more common.
1 and 2 are tied together and can create a death spiral. When you can't rent them, you don't cash flow as well/at all, which means things don't get fixed, which leads to reduced property value and you lowering rents, which leads to things not getting fixed..... etc.
I think you need to strike a balance: on one side, you can buy great homes in great locations that attract great tenants which saves you time and headaches, but the numbers won't be as good. On other side, you can go for properties with high returns but it takes more time and effort to operate them. Everyone picks where they want to be in this range depending upon what works for them.
Mike