A critical voice took the stand Wednesday in the first-ever federal jury trial involving Tesla’s Autopilot system: a safety expert who says the technology is defective and promoted in ways that encourage misuse.

Mr. Musk has said on many occasions since Autopilot was rolled out in 2015 that the system is much safer than human drivers and has claimed that Tesla’s technology was close to being able to pilot cars with no driver. Tesla uses the name Full Self-Driving for an advanced version of Autopilot, although it still requires drivers to remain attentive.