Problem is OP, that the 20th Century is absolutely vast in terms of the developments in warfare. It saw the introduction of smokeless powder at one end and the use of smart munitions at the other. Trying to write a statement about the efficacy of artillery over the entire span of that century is pretty difficult.

However, the old saying that

Artillery conquers, infantry occupies

Remains true even today. particularly if we view air power and smart munitions as ‘flying artillery’. The fact remains that only infantry can actually take and hold ground. Everything else is simply there to help the infantry perform their task at the lowest possible price, as per the following quote from John Monash, an Australian General from WWI

The true role of infantry is not to expend itself upon heroic physical effort, not to wither away under merciless machine-gun fire, not to impale itself on hostile bayonets, but on the contrary, to advance under the maximum possible protection of the maximum possible array of mechanical resources, in the form of guns, machine-guns, tanks, mortars and aeroplanes; to advance with as little impediment as possible; to be relieved as far as possible of the obligation to fight their way forward.

Its not surprising that this came from a Western Front General for this is the closest example of a pure artillery war that you will find. With such a concentration of men opposing each other on a constricted front, there was no opportunity for any flanking manoeuvres and there was no choice but to launch frontal attacks on the enemies fortifications – a murderous undertaking in any era, and futile in the era of barbed wire and machine guns unless the artillery could first demolish the enemy fortifications, and thus allow the infantry to hold and occupy the ground.

I have written of the British experience of artillery in WWI here and here. The TL;DR of which is that effective artillery, integrated into a well orchestrated all-arms methodology, was the key to victory; when the artillery worked, success was usually complete. When the artillery failed, the cost was appalling.