So: first: employers can't choose (freely) who to lay off. It's first in last out, or if you want to sidestep that, you'll have to compensate the employee quite heavily.
Second: "proper" union strikes have a very protected status. An employer who would hire scabs would be quickly taken out of business. They'd switfly be blockaded by other unions in sympathy. So for example: a store hiring scabs to replace striking union store workers would quickly find that their trash isn't picked up, that the cleaners don't show up, that no electrician fixes their fridges, and so on (Also: these type of measures are also protected).
Without having these two protections (can't fire, can't hire scabs) unions are very weak compared to employers. I'd argue that with these protections, unions and employers have a pretty equal playing field.
Note also that these protections didn't come free: the employers' share of the deal was that there is no minimum wage and you can't have protected strikes during the agreement periods which are typically 1-3 years. So the strikes will occur at known intervals if negotiations for the next agreement stalls.