santa4u said:
- It still allows employers to exercise the discretion of withdrawing I-140 / reusing labor and then open the possibility of two employees getting Green Card and potentially one guy getting rejected (which USCIS has not clarified upon till date?)
This is the big question with AC21. I can't claim any predictive powers over USCIS, but I would be personally quite nervous if I was the beneficiary of an LC that was previously used by an alien who claimed AC21.

Generally, USCIS has clearly prevented the Service Centers or District Offices from doing anything that guts the spirit of AC21. There's no guarantees that this will continue, but the clear intent of Congress was that AC21 relief could not be yanked away by the employer.
When somebody invokes AC21, USCIS at times issues NOID, that may potentially result in 485 denial and then the hassle of MOTR
First off, invoking AC21 does
NOT result in an NOID automatically. Second, USCIS occassionally makes mistakes, but you can't very well fix a process by merely saying "well, don't make mistakes".
The only thing that is guaranteed to result in an NOID is I-140 withdrawal, and in such a circumstance it is the only logical thing to do, since based on what USCIS has in evidence, you no longer qualify for permanent residence.
......USCIS should know that AC21 is more common and start working towards giving more time to employee to submit AC21 documentation and thus avoid scenarios of 485 cancellations to a minimal! 485 denial is so scary for an employee...USCIS should realize that and offer more time to an employee!
While 30 days may be problematic in some cases, I don't consider it an especially unreasonable timeframe. Evidence of your current job should be relatively straightforward (you do have an offer letter and a paystub, right?) and getting it to USCIS shouldn't take a month.