As far as the actual drive goes, I've been going down regularly at all times of day and night for almost 25 years and never had an issue, outside of a cow or horse crossing at night, though I've seen more of those on the US side. (Sells in particular.) This even includes an 11pm flat tire change on the side of the road heading towards the border on the Mexico side in the middle of nowhere. (Lesson learned, won't be heading back that late again!) The road leading to Penasco on the Mexican side is wider and usually better maintained than on the US side.
As for your vehicle's safety, just use common sense as you would in any public place, but it's not uncommon on any given weekend to see dozens of Hummers and tricked-out H2s, Mercedes SUVs, etc. If anything, thieves down there are more likely to steal a late-model Ford, Chevy or Toyota truck for parts than a flashy new vehicle that could easily be spotted heading out of town.
AND, the nationally-sponsored Green Angels repair service actually DOES go back and forth on the Mexican side with the intent on helping any stranded autos for free. AND ANNND, while you're at it, the best local insurance service with English-speaking assistance 24-7 can be purchased cheap and easy online at
http://www.proalliance.com.mx/