Don't they have the advantage of having very simple tissue?
Portuguese man o’ war is not a single organism at all but a siphonophore, a colony of many genetically identical but specialized individual organisms called zooids, all fused together and functionally dependent on each other.
Siphonophores are identical, except that they can be subdivided into units that are, much like organs, composed of multiple cells. Those units can be identified as evolutionary derivations of certain branches of the tree of life. This is more similar to your mitochondria, which are (like zooids) identifiable as having once been independent organisms, though unlike zooids your mitochondria are not genetically identical to you.
Actually, it bears pointing out that significant parts of you, like your blood, remain single-celled even though you're "multicellular".