The biggest concern is that a lot of heavy weapons have auto-fire, or even auto-fire only, and when the character has a decent pool the odds of getting neither net success nor 3 Advantage/a Triumph are very slim. Even the occasional miss becomes a hit anyway. With how already abused auto-fire can be, this buff makes it far too meta and takes away what is usually the downside to using auto-fire, because instead of a choice between higher risk to fail and a chance for multiple hits, it becomes the same or less chance to miss with a chance to get additional hits.
From a narrative standpoint, I initially thought the same way, but I’ve since learned more about automatic weapons, and so I’ll give a short description here of the various factors that I think make the RAW correct:
Controllability: Automatic weapons have rapidly repeated recoil. This makes it harder to keep the weapon’s sights on target, and results in a bigger “group” than if you were to pick and choose your shots.
Range: Smaller automatic weapons are lower velocity, use pistol cartridges (or pistol-proportional damage such as with the DH-17), and are only particularly accurate or useful at close ranges where you’re very likely to hit because the target is proportionally much bigger. Here, they are very effective, and in game you’re likely to Succeed with multiple Advantage or Success, either of which can be credited to the high volume of fire. The increase in difficulty makes sense for the loss of accuracy, because you’re less likely to land hits in critical areas because your aim will always be a little off. The actual relevance of that to real combat rather than hit-point/critical injury combat is at best debatable, but that’s a problem with the underlying system, not Auto-fire itself.
Doctrine: Larger automatic weapons, usually using rifle-caliber (LMG, light repeating blaster) or anti-material (HMG, heavy repeating blaster) ammunition are used as much for suppressive fire as for actual targeting. Machine gunners “paint” a zone, cutting down swathes of infantry or suppressing said infantry with the threat of cutting them down. Accuracy of fire or spread is actually not supremely important here because you want to cover a broad area. The M2 Browning .50 cal is actually very accurate, but it is intended for anti-material (e.g., anti-vehicle/aircraft) use rather than anti-infantry, for which a .50 cal is not ideal and a .30 cal is a more suitable choice (for reasons practical and logistical, mostly). Generally, higher rate of fire equals lower practical accuracy due in large part to controllability. Shorter bursts or selective fire (non-Auto-fire use of an Auto-fire weapon) will have greater hit probability for a single target. In the case of using the auto-fire setting at a longer range, success would more be a lucky hit, while Advantage on Failure would more be about Setback and what-not as you successfully suppress the enemy, keeping their heads down. What in game-terms is “select fire” is more likely to achieve both of those objectives.
Psychological: To an undisciplined machine gunner, firing wildly might seem like a good idea. “I’m firing so much, I’ve got to hit something!” However, that isn’t really true. Even suppressive fire must be carefully aimed, because to be effective it has to be within a certain proximity to the targets. More highly skilled and trained gunners will be more accurate in both auto-fire and selective fire, while novices picking up an automatic weapon for what might be the first time will be more likely to trust in volume of fire to hit their target and miss everything as a result. At close ranges, you’re unlikely to miss anyway under firing-range conditions, but throw in combat, cover, and movement, and those chances increase dramatically.
Just as an example, in flight simulators, many describe an increase in accuracy when using slower-firing cannons rather than machine guns because they focus more on actually aiming their shots since they lack the ammunition or rate of fire to try to cover the sky in rounds.
All that to say, I think the +1 difficulty is appropriate, especially if the intention is to hit multiple targets. If you’re trying to engage two separate targets in a combat environment, you’re less likely to hit either one than if you are focused on a single target. If you’re focused on a single target and trying to get a high volume of fire on target, you’ll either be trying to track the target with your burst (rather than firing a spread or a line in front of the target), which is very difficult, or you’ll be holding down the trigger for a longer burst, which results in a corresponding inaccuracy. Non-Auto-fire use of an Auto-fire weapon could represent a “spray and pray” method of targeting a single target, with the lack of a chance for multiple hits representing the large volume of fire over a broader area resulting in lower concentration of rounds hitting the target. It doesn’t have to mean you’re firing single shots.
So I agree with the RAW, but don’t like the addition of “Auto-fire (only)” to some weapons.