There's nothing inherently unsafe about buck-boost converters, they are used on millions of every day products, including the highest end of high tech stuff like motherboards and gpus.

If a circuit is in itself safe (it might not be if it's executed poorly), then safety of the whole device depends on how you're using that circuit.

100W is just as fine as 1W. It just depends on whether you're being scammed. If you don't have the knowledge to find out whether what you're doing is safe... don't do it.