It's impossible to assign complete fault to a single player in a team game.
I saw more games where Romo's play overcame others' faults than games where he was one of the players who deserved blame for the loss.
Let's use our 26-21 loss at Green Bay in the 2014 playoffs as an example. We all know about the call against Bryant, but the loss was because of much more than that.
In a game where the teams ended up being separated by five points, we had
1st drive, Romo being sacked on third down by Julius Peppers (probably the fault of the right tackle Free)
Green Bay drives for a touchdown (defense's fault, but hard to say who just looking at the play by play)
Dallas drives for a touchdown pass from Romo. Then, Mincey forces and recovers a fumble.
Romo throws another TD pass for a 14-7 lead.
Dallas forces a punt and the offense drives to the Green Bay 27, where Romo fumbles at the Packers' 38, picks it up and throws incomplete (can't tell by the play by play who was at fault, but might have been all on Romo)
Bailey has 50-yard FG attempt blocked after false start by Ladouceur (special teams blocking at fault)
This gave Green Bay a shorter field with 30 seconds left in the half and Packers drove for 40-yard field goal (six-point swing).
To start the third quarter, Dallas defense forces a punt, but Murray fumbles on first-and-5 from the Dallas 41 (Murray's fault).
The fumble leads to a field goal for Green Bay so we're at 14-13.
Dallas drives for a TD, but defense gives up TD drive to Packers.
Back-to-back sacks of Romo help force a punt (blockers more than likely at fault). Green Bay drives 80 yards for the TD and the lead (defense's fault). Dallas is now down 26-21 after the two-point attempt fails.
Another sack of Romo puts Dallas at third-and-11, but a 9-yard pass to Beasley sets up the fourth-and-2 "incompletion" to Bryant (Bryant at fault for not making sure the catch was secured, or officials at fault for not upholding the ruling on the field).
With 4:06 left, the defense allows Green Bay to run out the clock (defense at fault).
So the reason we didn't win (just based on looking at the play by play) was the offensive line, the defense, special teams, Murray, Bryant and possibly Romo. Anyone who puts the blame on Romo just blames the QB for any loss, and this could be done with pretty much every loss, with Romo maybe deserving more blame in some than he did in this one but there also being multiple reasons that we lost.
Blaming Romo for this loss would have been akin to blaming Rodgers if the call had not been reversed on Bryant's catch (and the defense had been able to hold if we scored, which I doubt would have happened since Green Bay would have just needed a field goal).