I think being a landlord is like everything else, there are upsides and downsides, and there are "amateurs" and "professionals".
By which I mean that being a landlord is a business, and it takes time and experience to learn the ins and outs of any business, and ultimately experience improves performance.
By extension the landlord with one property, gets little overall experience (maybe one lease a year or so) so they encounter all the rental-business-challenges, one at a time over a long period of time.
It may take years to get a bad tenant, or years to figure out what is the correct amount to spend on maintainence, or discover what kind of garden works best, and so on.
With only one property under management, there is also magnification of the effect of individual events. One property, and one bad tenant, is a big deal. One property and the need for a new kitchen, of bathroom etc is a big deal.
A portfolio of multiple properties helps to spread the load. 1 bad tenant is only 10% of income and so on. Plus with 10 properties you can have a regular plumber, electrician, handyman etc, and so you get good rates, and good service, because you become important to them.
Your family member has learned an expensive lesson about vetting tenants, that lesson will cause some to exit the market, and others how to "understand the landlord business better."
Typically it makes landlords more risk-adverse, and this has knock-on effects for those who are indeed bad tenants. (for starters the only ones who will rent to bad tenants are inexperienced (bad?) landlords.
The spill over of this are folks who have flags (previous bankruptcies) who may be perfectly OK now, but are victim then to landlords that aren't prepared to take on that risk.
Some larger landlords can take on some percentage of "medium risk" - small landlords will learn (the hard way) to only take on "no risk".