In that case you are NOT being unreasonable- the LL must know he would have a problem re-renting the property once the neighbour problem was known to prospective tenants, he shouldnt expect you to lie for him just so he could get a new tenant.
However the reason why I asked is this
A friend has a house he rents out, perfect family home, nice neighbourhood (well it was but read on) good schools, large garden, reasonable rent.
He had a family who lived there for 6 years, and quite frankly would not have had a problem if they had wanted to live there forever they were what he wanted a family who could make the place their home.
Just over a year ago they just gave notice but with no reason, even though he offered to lower the rent they wanted out.
The next tenant only lasted 6 months, when new tenants were shown round
the existing tenant told them there was a BROTHEL next door (I kid you not) - the house was occupied by three ladies of negotiable virtue and had been for about a year.
My friend knew nothing about this presumably the first tenants left because of this but hadnt told him, the second tenants did not bother to tell him only a prospective new tenant.
As soon as he knew he did something about it - ended up with him having no luck with the police but he found out the name and address of the LL who owned the property and had a LL to LL chat and the three ladies were evicted (eventually)
If he had know about the problem as soon as it became apparent he could have acted sooner and possibly kept his original tenants, as it was he lost
about 5 months rent whilst the eviction was sorted out but that was better then not being able to rent it out at all.