Actually, they are definitely unreliable. I've received a total of 8 RTD's throughout my asylee/PR status. I've sent them exactly the same way every time. Processing periods have varied from 4 weeks to 6 months.
I received my 6th RTD with a receipt processing time of 950-999 days; received the RTD in 6 weeks & 2 months before the projected NSC processing time.
I know of at least four people who received their RTD's in the past 2 months with Receipt Notices dated anywhere from November 25th, 2003 to February 5th, 2004. No expedited service requested/nothing special or different done.
Bottom line - it's a crapshoot! It all depends who gets your case. Some officers are dilligent and speedy; some (probably hired through affirmative action practices) are lazy and slow.