Scar/ Voice chip
To me, these were the 2 biggest points of contention in this movie in regards to the true identities of Castor and Archer.
I think that it would have been better if they did not add the scar to Troy or put it on the wrong side of his chest. So that when he is undressed, Eve would have noticed that something was not right. However we never see Castor's bare chest when he is assuming Archer's identity so we don't know if the scar is there or not.
I just felt that the "reveal" could have been better or at least more clever than the blood test. That was a bit too over-the-top in my opinion.
Also, they never touched upon the voice implant in the movie. The doctor says that it could easily malfunction due to trauma or even a violent sneeze. Archer gets beat up dozens of times yet there was never really any moment when his identity was in peril in front of Castor's criminal underlings. It would have been interesting to see his voice "screw up" at some point.