and even if you file 485 you can always stay

on H-1B without applying for EAD. EAD gives you the flexibility to change employer after 180 days after 485 filing, given that your new job is in similar field. However, once you use EAD, you lose your H-1B status, so in case 485 gets denied for any reason, you will lose your legal status in the US immediately. This means there's a tradeoff you need to make.

I don't know how EAD works if it's based on a family based 485.