It seems to come down to if you believe they are 100% breathable or not. The EU safety testing standard says they are but if they aren't breathable, then they would be less safe than using a mosses basket.
I think I'll just test it myself and see how breathable it is when I see one in real life. Without any specific reason to doubt the EU safety standards, a personal real life sanity check on the product will probably be enough for me to decide.
I wouldn't use Bumpers though, even if they are breathable. Even when breathable (which most are now), they still pose a strangulation and falling risk.
If fact, even before the US design requirements were changed, most of the deaths (in the US at least) were from falling and strangulation rather than suffocation. At its peak, there were around 7 deaths a year from suffocation from the old thicker bumper design (out of 4m births a year). While every death of a baby is tragic, the risk of suffocation seems exceedingly low even for old style bumpers.
However, with new EU crib / cot safety design requirements, bumpers also no longer serve any practical purpose.