Example:

$500,000 purchase, 10% down

2.875% Fixed
Monthly Payment $1,867
7 years interest payments = $83,520

4.0% Interest Only
Monthly Payment $1,500
7 years interest payments = $126,000

You save $367/month, which totals $30,828 over 7 years

If you were to reinvest that cash flow savings instead of locking it up with the bank, the 4% interest is less. Plus you realize the same appreciate gains with an interest only as you do a fixed.

Also, you can only get so many conventional loans before you're tapped out. A private lender doesn't base things off your personal, so as long as you have a good deal then there's really no limit.

I understand that there is risk, really comes down to your comfort level and being able to sleep at night.

The only downside if interest rates skyrocket at the end of the 7 years. But who knows if we'll even be alive in 7 years.