I've got two P320's one's a .40 the other a VTAC in 9mm. I've only put about 150 rounds through each. As of yet I haven't had any issues with mine. However neither of mine have a safety and a loaded chamber indicator as does the M18. Whether that had anything to do with the reliability of the M18 I couldn't say? I changed the grip frame on my .40 to a Wilson Combat. I have an earlier model that Sig had a voluntary recall on due to drop safety issues.
I added an Agency Arms trigger that has an integrated safety like a Glock to mine. Apparently the original all steel triggers were too heavy and when dropped the shock combined with the momentum of the heavy trigger caused the gun to go off. Sig solved the problem by changing the trigger, sear, firing pin safety lever and added a disconnector.
At least in my opinion they could have just changed the trigger to an all aluminum one with a trigger safety like the Agency Arms trigger? There's no way in hell that gun's going to go off accidentally when dropped with that type of trigger installed. Now it's really no different than a Glock or any other striker fired gun equipped with a trigger safety.
I don't know what's going on with Sig these days? They should never have let those early P320's leave the factory that way. In spite of that they are good shooting guns, so far I've been happy with mine. But by all means rent one before you buy one.
The P320 X5 Legion by all indications has been very reliable and has been very popular amongst competition shooters. I didn't want the Legion version as it's too damn heavy for everyday carry. They have tungsten infused polymer frames which makes them as heavy as a steel framed gun. Sig also sells those frames as an accessory. But at around $250 they're not cheap. My Wilson frame was $65.