The concern is aboout about the job. People are worrying about if I-140 is rejected, they will lose all valid immigration status. They have to leave the US.
If you have H1B status (not AP), you are actually maintaining dural status: one is I-485 pending, the other is H1B. If I-140 is rejected, they can still stay in U.S based on H1B. But if they use AP, you only have one status. You can still work based on EAD,and change job based on EAD, but not on H1-B visa.