The way I would personally explain it is that Unions are great for rallying workers behind a party but not for seizing political power themselves. Anarchist Spain was one of the few countries that went the syndicalist route and ended up being isolated from the masses. A vanguard party that Unions rally behind allows for it to conquer political power. Anarchist Spain didn’t have a party and just an amalgam of unions; strong, but without the necessary mass support. What made the Soviet model so unique is it put workers and peasants in a position to self govern while also represented in parliament by the RSDP. The CNT-FAI did not conquer political power and in effect distanced itself.
On top of that, not all unions would be radical and I’m talking about back then. Today there are very few radical unions and they wouldn’t come close to conquering political power in any way.
Anarcho-syndicalism falls to all the same issues that vanilla anarcho-communism does.
How can one expect to fight off reactionary influence both from within and without if one doesn’t have a decisive central power to help defend the working class and solidify the revolution’s hold?
Anarchism has existed nearly as long as concepts of socialism and yet there have been numerous successful ML revolutions and not a single successful anarchist one.
Changing the organizational specifications won’t change these things, syndicalism is no more potent or effective than average anarchism.