We both work as realtors, so we have some flex in our schedules.The downside is the investment of time it takes to manage projects, especially those that involve square footage additions or major reconstruction.

So it has its downside.

@Andy Nguyen the only downside is that you have to be very selective on which apartments, some landlords try to charge you more a month because you are doing airbnb and then you can't justify the margins after furniture, insurance, etc etc.

There are some great upsides, and then again their are some downsides.

Check if the electric & gas companies that service your rental have "landlord protection" programs that will auto-flip utilities into your name in lieu of shutoff.The only downside to these programs is during summer months when tenants fall behind on utility bills, you get stuck paying.

Just keep an eye on the downsides – more debt and the risk to your primary residence.

However, I don't see a downside of putting one down, since it will only be a fraction of the cost of the whole project. 

The downsides typically are that you're maxed on contributions and loans before it becomes a MEC and loses its tax advantages. if you're even remotely interested in something like this, speak with a qualified Financial Planner. 

The down side is mostly time constraint, and any unforeseen snags along the way could cost a lot more time and money.

The only downside to chase is the FEES!