The wisest real estate investors I know diversify across Asset Class, Geography and Operator.
Even the successful active investors, house flippers, self managing single family and small multifamily owners are wise to eventually diversify their portfolio into more passive diversified assests like syndications and mutual funds.
Asset Class: Multi-Family, Self-Storage, Mobile Home Parks, Assisted Living Facilities, etc. (four asset classes we prefer long term for a lot of reasons). If all your portfolio is in commercial industrial buildings and there's an event that severely impacts the demand for commercial industrial, your heavy exposure to that one asset class could really hurt.
Geography: Dallas, Cincinnati, Las Vegas, Phoenix, Jacksonville, Columbia, Birmingham, etc. If you aren't concentrated in one region of the country, you're less likely to get wiped out by a regional black swan event (weather event, economic slow-down, etc.)
Operator: There are a lot of good syndicators out there, there are a lot of bad ones. How good are you at picking out the good ones? We get a lot of repeat businesses from our investors, but even our happy ones eventually start rolling part of what they've made with us into other syndicator's deals. Even a good syndicator could have something unexpected happen. If all your portfolio is tied up with that one syndicator/operator, his misfortune could become your misfortune.