Common advice is you only have to be good at either hiring slowly or firing early.

Google, for example, was apparently very good at hiring (slowly) - at least in their early days. So it didn't matter that they were (are?) terrible at firing.

Facebook was the opposite AFAIK. It didn't matter that they hired too soon and too many of the 'wrong' people - because they were good at letting them go before it did too much damage to the business.

On balance, I'd rather be good at hiring than firing, as it seems to be much more fair on employees.