Consumer products are guaranteed to be safe if they have the appropriate certifications.
Since they show no certifications on their site, for example on their product page or on the FAQ, I've asked their customer support and this is their response.
Which is, in my opinion, quite suspicious, because on their website they claim:
All the authorized Earthing products are specifically designed for safe, biological grounding of people. They have a built-in resistor that limits the flow of current in order to prevent the unlikely possibility that electricity would flow through the connecting wire and possibly hurt someone.
Either their product is electrical, and they need to provide certification, or it isn't, and then it should not need a resistor and other systems to prevent getting electrocuted.
Of course, do take notice that while you have to trust that I received that answer and I am being honest, you can check for yourself by using their contact form.
Given the self-admitted absence of certifications to guarantee their claimed safeties are adequate (or present!), I consider it better to default to a caution position:
In theory, assuming wiring is done right the ground prong will contact the ground terminal in the outlet and that terminal is connected to the ground with a thick enough wire and touching it is as safe as touching a washer or fridge chassis.
However you should not do that ever, because Murphy is out there...
Fact, which, incidentally, is repeated on the web site (except, to convince they are different and safe).
In conclusion, there are no provided international guarantees that it is safe. However the company contends they are not needed. Use your best judgement.
That is the focus of another question: it's disputed.
Also see Grounding & human health – a review, I A Jamieson, S S Jamieson, H M ApSimon and J N B Bell, for a meta review which shows inconclusive results.