How to respond to people living in home who are not on the lease?

[Jonathan Taylor Smith]

I've got tenants at 3 separate locations who appear to have other people living in the homes who are not on the leases. But the homes are well kept (inspections passed), no problems have been caused, and rent is paid on-time... So why should I care about the other person(s)? Should I inquire about this and push the issue, thereby risking a conflict with these otherwise perfectly fine tenants?

All 3 of these tenants have Section 8 Vouchers, so it is likely they cannot list the additional persons for that reason. In one case it appears to be a boyfriend who is present more often than not and the lease is coming up for renewal. In another case also up for renewal it is a brother, whom I've meet. Without having run a background check on him, he would likely be an acceptable resident. The 3rd case is for a new tenant (2nd month) and it appears to be the tenant's parents, who may be living in the home with or in place of the leasing tenant.

I've heard that should an eviction be needed, having person's living in the home who are not on the lease makes that more complicated - but not sure why or how exactly. And I'm aware that this situation could allow a person to be present who I would not have otherwise rented to... But is there some other reason(s) to rock this boat and risk creating vacancies out of tenants who appear happy to stay for years? And if so, do I send a notice to the tenants or just inform my Section 8 rep and see how they respond?

My landlord experience began in 2015 and thankfully I've yet to evict a tenant or have any serious dispute. I've even been able to return near full security deposits to those tenants who have left, as my homes have all been treated with care!