I've often read such claims, but I would think that a safe and reliable machine gun would be a bit more complicated. If cycling the action disconnects the sear from the trigger, there isn't much risk of the hammer being "barely" caught by the sear as the gun cycles, but preventing such a thing in a full-auto firearm would be much harder. If the hammer barely catches on the sear as the trigger is being released, it could leave the gun in a situation where an arbitrarily-small nudge could knock the hammer off the sear. If the firearm has a backup safety mechanism, the it could be unexpectedly decocked. If it doesn't, it could unexpectedly discharge. Neither seems like desirable behavior.
I would expect that a reliable full-auto design should include some timing mechanisms to ensure that the hammer will only be allowed to approach the primary sear at times when the action is either committed to firing another round or committed to stopping until the trigger is pulled fully. Not exactly a 10-minute mod.
What’s your opinion of the MG-42 under battle conditions during the war? They were used by the Germans to good effect on the eastern front, or so I’ve been told. But I do remember reading a book by a German veteran of the eastern front who was a machine gunner and he claimed that in the last 18 months of the war the MG ammo they were given caused the weapon to constantly jam.