I understood this...slightly
https://viralzone.expasy.org/5957?outline=all_by_species
Syncytia is formed by fusion of an infected cells with neighboring cells leading to the formation of multi-nucleate enlarged cells. This event is induced by surface expression of viral fusion protein that are fusogenic directly at the host cell membrane.
Syncytia can only happen with viruses able to directly fuse at the cellular surface without the need of endocytosis.
Dave Barry shtick: “endocytosis would be a great name for a band!”