Risks of letting to a guaranteed rent agency

[MrsPicklesonSmythe]

Does anyone have any advice or words of warning when it comes to dealing with an agent that guarantees your rent? The one I'm looking at effectively become your tenant then let it out paying you whether they have tenants or not. They pay a slightly under market rent but haven't mentioned any other drawbacks.
Is there anything I should be concerned about?

I found this article online:

www.telegraph.co.uk/finance/personalfinance/investing/buy-to-let/10718950/Landlords-warned-of-rental-guarantee-pitfalls.html

In the worst cases, rooms will be let to multiple to get highest yields. So a three-bedroom house could have six or more people living in the bedrooms plus more living in the reception rooms with only the kitchen and bathroom unoccupied. Apart from the fact that most decent LLs wouldn't want to be part of this sort of set up, however unknowingly, the property itself will face enormous strain accommodating that number of people and will become unsanitary and tatty. I've heard of cases where the property has been let for a fixed period of several years during which time rooms are subdivided to create an illegal warren of "bedrooms".