You are wrong about the burden of proof here, there is actually more reason to be suspect of the widespread use of ropes as opposed to nails, based on the factors I had elaborated on above. Although you still believe that nailing through the wrists was "unrealistic", a wholly ludicrous complaint based on a misunderstanding of the anatomy of the upper arm and the mechanics of blood flow throughout the body.
Fine. I can just look through original sources.
In a private discussion recently a friend of mine recommended the play "Mostellaria", by the Roman author Platus, so I had to look into it myself, and sure enough there is a reference to crux (seeing as their are many slave characters in the play, and it is set in Athens). Act 2, Scene 1:
Égo dabo eí taléntum, prímus qui ín crucem éxcucúrrerit ;
Séd ea lége, ut óffigántur bís pedés, bis bráchia.
"bís pedés, bis bráchia"; that's an exact fucking quote. Two for the arms and two for the feet. That was about 210 BC.
Here's the link to to the pdf of the book on Google, you can look for yourself.
Nails were never in short supply. Valuable, yes. Rare? Almost never. I will actually quote my friend Jeddak at length here:
"When the Romans abandoned their outpost at Inchtuthil in Scotland in 86 AD, they left a hoard of over 875,000 square nails along with other iron objects buried, apparently underneath a building which they then burned to further disguise and hide the site. The hoard was found in 1960, so their ploy worked. What it shows is while those nails were valuable, they were not indispensable, not even worth the cost of the labor needed to haul them back south. Had it not been for the hostile Scottish tribes, there might not have been a need even to bury and hide them as they did."
I've heard of the hoard there before, especially referenced in our own "Roman Resources" thread, but still, it presents a very striking picture. Nearly a million nails! Enough nails to crucify 200,000 people, truly a ridiculous number. But we do know that the Romans did crucify en masse in that way. Thousands at a time, even. Remember, the after the third servile war, the revolt of Spartacus, the executions along the Appian Way? The siege of Jerusalem? The crucifixions of the slaves of Pedanius Secundus? All involving thousands or hundreds of executions. It is unlikely that every single cross could have been so carefully guarded as to prevent escape if simple ropes were used. No, they must have been secured with nails, through the heel or foot, and the wrist.
"bís pedés, bis bráchia"
So my question to you is, which one did you think they nailed first? Feet or wrists?