A long time ago I had a paid subscription as a potential 'walker'. I wanted a dog but didn't have the lifestyle to look after one full time because I was working.
I found it absolutely useless because I wanted to 'borrow' a dog when I had time - in the evenings or at the weekends but most owners wanted to spend that time with their dog and were looking for free 'doggy day care' all day or to leave the dog full time with the borrower for a holiday - but if I could have had a dog full time, I'd have had my own!
I met a couple of owners and dogs but never got involved in a regular set up as neither of them suited me. The one owner was an arse and the other dog wasn't my cup of tea. I am a careful and responsible purpose who just really wanted a dog and thought this might be a fun way to 'make friends' with a dog I could walk regularly.
When you say 'is it safe' I assume you mean are people going to steal your dog?
Like I said I was on it and perfectly normal non-stealer of dogs - but gave up as found it useless. I expect it's like internet dating and you need to use some common sense - speak first, meet the person with your dog, find out about them, verify their identity, maybe look for people with paid subscriptions - at least you know they've provided a credit card to do that.
But if your dog is a pedigree expensive Best of Breed type with chunky value on the open market, then you'd probably be better off paying a professional walking service.