This situation can be complicated for several reasons, the first of which you've already mentioned.
1. Eviction - It can be exceptionally difficult to evict someone if you have no direct contact with them nor know their name or who they really are. We didn't necessarily need to evict someone, but even trying to tell someone that we will not be renewing the lease because we plan on remodeling the property is difficult.
2. Displacement of tenants - Let's say for instance that your tenant who's boyfriend has appeared to move in, does actually live there and you are inadvertently accepting funds from the boyfriend. If for whatever reason the two have a falling out and your tenant decides to move, you technically do not have an existing financial contract with the boyfriend.
3. Last and most potentially risky are legal issues - This is true for friends visiting the property as well, but there are sometimes greater legal risks with more wear and tear on the property. Let me unpack that. What I mean is along the lines of you will more than likely finish a puzzle faster with more eyes on the puzzle. More time, more bodies, more everything on a property opens more possibilities for risk/exposure. For example, we have a tenant who moved her boyfriend in without telling anyone and works from home and washes down the stairs every day after taking her dog for a walk. I have heard more from this single tenant than every other tenant combined. She has issues with bird poop outside, stairs chipping due to water and impact, puddle in the garage from sprinklers (it's the size of my cell phone and not always there). Don't get me wrong. This tenant is unique that way, but if you can eliminate the risk, it's nice to sleep well at night.
I think it is several minor "gray" issues that could lead to far more significant black or white concerns. I typically have a clause in the lease indicating that if someone has been staying/living in the property for more than 3 weeks, I need to know otherwise there is a financial hit to the rent totaling roughly a late fee. To me, land lording is all about hoping for the best and preparing for the worst. The next tenant will always bear your burdens of your last tenant because of the lesson you learned the hard way. Up to you and how you choose to handle it.