Investing in rentals is definitely not a fool-proof clean 7-8% a year. There's the potential hassle that TraderZones mentioned, though it's a lot easier if you have an agent/manager whom you trust to handle a lot of the footwork, finding and dealing with the tenants, etc. Of course that will cost you something, usually a month's rent as commission, and in general the taxes/costs/fees have a way of adding up. It's not free money, but if you get a good deal you can probably make 5% a year on average, plus the benefits of any appreciation over time. I've never owned an actual building (just individual units), and there might be added considerations. If you have more control, maybe you can reduce overhead somehow or cut costs to squeeze an extra 0.5-1% per year out.
I would say any agent would cost you at least 10% of your take, so you would be negative assuming "7-8%" And many agents have a habit of doing the minimum. When I took over a property, the tenants said the management company was poor...