Worse than land mines. The land mine goes off once and is done with its killing/maiming. The "weapon" can keep killing until its magazine is empty and may be mobile to search out targets. The location land mines are deployed in can be known. A mobile autonomous weapon endangers a much larger area. If it can repower/refuel/reload itself the endangered area is much larger again. If it is intelligent enough it could use other resources to multiply its killing power such as firing weapons at targets to cause explosions (refinery tankage for example or tanker trucks). Intelligent enough and it could fight a guerrilla war lasting months or years.
Over the long term the danger of AI is our turning over the means of production to them then "provoking" them into attacking us. They need not even have self awareness or awareness of us as people, just have us be a problem to be solved where violence is the discovered solution.