Assuming that they’re keeping skin in the game on each property and initially paying cash, how is serially buying investment properties using lines of credit any worse than having cash on hand and paying 20-25% down for them?

Answer: it’s not. And as long as rents exceed monthly expenses by a tidy margin, it’s a relatively safe thing to do provided that the credit lines can be converted to fixed-rate loans within a reasonable time.

Flipping would be fueling the bubble. Buying for rental income and remortgaging below purchase price is just good business sense.