Not as much ammo as machine guns.
True, but they also don't use as much.
With a vehicle mounted system rigidly mounted and with optics and ballistic computer I would say the grenade distribution would be much much less than from a portable grenade launcher... they tend to jump around a lot when they fire leading to a spread of grenades on targets at extended distances.
Shooting at a target at 1.5km a 5-10 round burst of grenades would likely be rather effective, while a 50-100 round burst of 7.62 might not get a direct hit on anything.
They are heavy weapons and the grenades are much more costly and take longer to be produced in less quantity than stabdard ammo.
The ammo certainly is much heavier and more bulky and also rather more expensive, and against some targets might not be as effective as bullets (especially AP bullets or other specialist types...
However precision at long distances isn't that great. Plenty of footages shows it being used as a mortar to saturates a zone or some random shooting hoping it will touch something.
Another factor is range, with the new 30mm grenades (well they aren't that new now) their 30mm grenade launcher can reach 2.1km which is rather better than rifle calibre machine gun fire which would probably reach its limit at about 1.5km unless you use very long bursts.
Their new 40mm grenade launcher fires to 2.5km and has a bigger heavier more effective grenade.
The crew-served heavy grenade launchers are basically pocket mortars.
During WWII a Soviet designer actually had a 50mm automatic grenade launcher intended to replace the 50mm mortar... the idea like the newer models was to make up for the small bomb by firing them in numbers giving a cluster bomb effect which made it more effective, but the designer died and the project disappeared... most military forces eventually came to the conclusion that 50mm mortars were just too expensive for what they were and dropped them for rifle launched grenades or nothing at all.
The lack of precision is intended. You want that weight of fire spread over a large area instead of churning up dust for the same patch of ground.
The ground based launchers certainly jump around during firing, but the effect of the grenades is scary... there is a Combat Approved episode showing them testing the 30mm grenade launcher with cardboard and balloon targets showing the fragmentation extent of each grenade.
And the grenades are bouncing after hitting the ground, explosion happens somewhere between 1,50 and 2,00 metres.
They don't naturally do that, but the Soviets/Russians have a model of 40mm grenades for their underbarrel grenade launchers that has a separate bounding charge to blow the grenade up into the air before the main charge explodes... it detonates the main charge usually at 1.5-2m height.
With standard impact grenades they tend of course to explode on the ground resulting in serious leg and lower body injuries, but with the bounding grenades (idea based on german mines that did the same thing) means they explode at head height so the wounds tend to be head and neck and upper chest wounds that are far more dangerous and life threatening.
Much better bang for your buck.