The interest rates you would get from hard money would likely be WAY higher than if you financed with a traditional lender.

Any scarce item has greater likelihood of increasing more in value over time.

With the appreciation rate the way it is in San Antonio these last few years, I've even seen some clients buy traditional buy and hold deals and be able to cash out refinance within a year or two when they've bought right.

I love the numbered list that @Chris Seveney laid out for you, it's a nice checklist of items you need to know. 

Considering the big chunk of change that is even 3%-5% down for houses at Boston prices, I wonder if it may be worth it to buy in a neighboring town (within a 1hour driving radius) where prices are notably lower, put a more traditional amount down (10-20%), and just rent out fully without house hacking.

Considering the big chunk of change that is even 3%-5% down for houses at Boston prices, I wonder if it may be worth it to buy in a neighboring town (within a 1hour driving radius) where prices are notably lower, put a more traditional amount down (10-20%), and just rent out fully without house hacking.

My understanding of loans and financing is pretty limited.The latter scenario you describe of financing the purchase and rehab and then performing a cash-out refinance is quite frequently practiced.Often, an investor will get a loan for the purchase and rehab of the subject property, and then (after seasoning) cash-out refinance using a traditional bank loan.

I used this in one of our recent properties in the 'kids' bathroom and it looked a little more unique than traditional 3 x 6.

My partner and I have enough for the down payment on a flip, but we’re wondering if we are better off using a HML vs traditional financing. 

This method will likely give you higher cashflow than a traditional rental.