I have used Airbnb in Europe and not had any problems. Be as cautious as you would be booking anywhere - look for the number of reviews, length of time on there etc.
How are you going to get around and how long do you have in each location?
If you are going late May, the weather should be good, but maybe more unsettled earlier in the month. There is plenty to keep you occupied in Lisbon, Sintra and Porto. Take a boat trip in Porto and a tram in Lisbon. I was surprised how hilly Lisbon was, so wear good shoes.
I was in Salema for the last 2 weeks of May and the first week of June 2015 and the weather was hot, but locals said that it was hotter than normal. Due to the wind in Salema, we didn't have and didn't need air conditioning. Salema doesn't fit easily with your other locations, which is why I asked how long and transport questions.
I have had c 15 trips to the Algarve over the years and it has changed much in that time with lots of new tourist apartment developments. Salema is such a place. We found that both the supermarkets had closed down in 2015, so the nearest is a couple of miles away near Budens - this was OK because we had a car. In the whole 3 weeks we were there, we didn't see any small fishing boats go out as we have previously and the men weren't sat there mending their nets. The bread van wasn't in the village in the mornings as it has been previously (maybe it was there in high season). Half of the restaurants had changed hands in 2 years - not a good sign. Worst of all was the huge white elephant on the hillside that was supposed to be a 115 unit tourist complex. The developer went bust 4 years ago when it was part completed and it sits there gradually declining. Maybe something has happened since last May, but...... All in all, Salema is no longer the quaint fishing village that it was when I first visited it. We still had a good holiday, don't get me wrong, but it has changed.
Be careful with your belongings. The only time I have ever experienced any crime is whilst in Portugal - a break in to the apartment and someone attempted to jemmy the car door lock. We now ensure that nothing is on display in the car and leave the glove box door open. We are going to the eastern end of the Algarve this year, so none of this has put us off - just take care as you would anywhere.
Book your accommodation ASAP as Portugal is likely to be busier this year now that Tunisia, Egypt etc are out of bounds for Brits and others due to the terrorist threats.