@Sarah977
You are absolutely right to advise caution, especially as @Kanani2's sitution is very different to mine, being a complete unit.
Here, the law is also very strict about tenants' rights (completely different to lodgers' rights) and we can't just throw them out either. It involves a long process and a loss of income, not to mention the stress.
I once had a very bad experience with this myself. I had foolishly allowed some friends of a friend to temporarily stay in a very nice flat I had just redecorated and furnished ready for rental, providing they would contribute a certain amount towards the bills, which they said they would. When I say temporarily, I mean for one week (then agreed to let them stay for two) because they were in a fix and told me they were planning to leave the country.
Well, five months later, they were still there, having not paid rent nor bills and clearly showing no intention of leaving.They had kept a huge dog there without my knowledge (also putting me in breach of my lease) destroying what parts of the flat they didn't destroy themselves, taken copious amounts of drugs and caused the neighbours to file official complaints.
I tried to reason with them, but they were the type of 'professional tenants' you refer to and I realised they had surely done this before (the girl got fired from work for stealing and after they left there were endless demands for payment from various companies dropping through the letterbox).
I had to consult a solicitor and luckily they upped and left when, on his advice, I sent them letters by recorded mail. Luckily, I say, because it would have taken me at least two months more and who knows how much in legal fees to get them out. I was already thousands of pounds out of pocket (it affected my credit rating too) and deeply stressed by the whole experience.
So yes, be cautious about any long-term letting situation and vet, vet, vet your tenants as far as possible. However, I'm not sure that one should rule out Airbnb guests for this reason. I don't think they are riskier than any other kind of tenant On the contrary, I find them to be more respectful. Plus, you CAN have a rental agreement with long-term Airbnbers too.
The only downside is not having control over the deposit. However, in the UK, a regular landlord doesn't have control of that either as it has to be put into a deposit scheme and, like Airbnb, someone else has to approve any claims.