Yes, those are two obstacles that employers face when hiring strikebreakers, along with more significant obstacles like laws forbidding strikebreakers, licensing, and union violence.
The common factor is that it's not enough to refuse to work, unions must be able to ensure that no one does the work.
> unions must be able to ensure that no one does the work.
Often the barrier is natural, such as with folks at Google who built the systems and are (collectively) the only ones who can keep them running without significant interruption. By what authority, for what compelling reason, would you abridge their freedom of association (or in this case non-association)?
I wouldn't, they should be free to work or not as they choose. But they shouldn't be able to abridge anyone else's freedom and I doubt they're as irreplaceable as you think.
Doubt all you like. It doesn't matter. What matters is whether Google doubts it, and I think they know better. They know the costs of hiring and training and downtime for their business. Do you seriously suppose that they think they could just find a few thousand replacements on a moment's notice?
Unions are rather weak when they can't prevent employers from hiring strikebreakers.