This thesis statement was driven by the collective understanding that there are significant rental inventory supply issues in most markets in the country.

People understand that it is possible, but takes a lot of capital.You can still buy cash flowing properties in Milwaukee, but I caution everyone from pushing this to the extend possible: you end up in the hood with headache tenants, rent colection issues and old delapitated buildings that need massive capex. 

Whenever you are using leverage, you need to have ample slack in the system and sufficient cash reserves (or access to cheap & instant cash) to cover vacancies, capital expenses, unplanned maintenance, and other issues to avoid missing your note payments. 

I'm not proposing to change the landscape and what the properties I do have noise and parties have not been an issue. 

What other issues/roadblocks may I face that I am not aware of?

Some things may be different, such as the occasional leasehold condo and coastal issues, but the rest is just the fundamentals -- marketing, financing, rehabbing, etc.

I try to spend a little more time initially on making sure the maintenance is up to par when I buy a property and I rarely have any maintenance issues.

Where I lack understanding is at what point do you start making money for example you buy a duplex lets say it was 300k the monthly mortgage is 2,500 each renter pays 1,500 thatd 500 cash flow which is I guess “profit “ but as the landlord you're responsible for major repairs such as but not limited to roofs, plumbing issues, electrical issues that all can be expensive you would have to have money saved so that 500 cash flow is profit but wouldnt it just have to sit in a bank account? 

This is the best place for all things real estate related.

Usually HELOCs are for short term purposes so it would make sense if you can generate enough STR income to pay off relatively quickly