This is a great idea, recycled plastic melted down and formed into blocks. And they are pretty and the variety of colors makes them very artsy.
As an engineer I can tell you that the strength and durability characteristics are likely better than standard construction blocks. Unfortunately, that “likely” means that they will not like be used for buildings since the strength depends on the kinds of plastics that are found in the garbage.
I hope to see this in Home Depot as a novelty item as some point but it reminds me of effort to used French Fry oil from McDonald’s in place of diesel fuel. Sure it works fine, but the quantity of French Fry oil, really isn’t enough to power a city. Plastic is everywhere but your average grocery bag melts down to a little tiny dot.