I live 30 miles from the Mexico border along multiple heavily used smuggling routes. I hike and backpack frequently in mountains used by those smugglers and have encountered groups of smugglers many times over the last 7+ years. I have never seen them armed, nor have I ever felt threatened by them. In almost every encounter, they get off the trail and move away before we get close, or if they get startled and there's no time to really do that, they just sort of keep their heads down and pass quickly. Everyone I know who hikes in the region, including a former Border Patrol agent who worked this area for 6+ years (and never saw an armed smuggler), has had a similar experience.

Edit: There are some areas along the border where smugglers are reported to be armed more frequently, but this is almost exclusively in areas where two cartels compete for smuggling routes and their weapons are carried to defend against the other cartels, not to use against civilians or law enforcement. They don't want to get caught with weapons in the U.S., as that significantly increases their legal exposure vs. just bales of marijuana.