Ghost Rider is undoubtedly is among the most powerful of Marvel's demonic heroes. Johnny Blaze, a once-common motorbike driver, acquired immense powers as he was tied to Zarathos by a contract with fiend Mephisto. During the influence of Zarathos, Johnny became a "SpÃritu de Vengeance," a skull-faced fiery motorcycle, able to do anything.
But while, with this unholy connection, Johnny gained incredible strength, it would come as surprise if Zaratho had gotten the short end as his demonic power had been lessened by Johnny Blaze. It sounds odd, but Blaze's biggest contribution towards this demonic superhero can really be his ability to avoid the rush of his evil side.
Those who have seen the Ghost Rider in operation could find it difficult. After all, when Johnny Blaze transforms, he often thinks that there is nothing in his way, not even magical beings. The Ghost Rider has immense superhuman strength, can control torrents of the infernal fire capable of carrying out human souls and is wielding the frightening "penance stare." This power alone is so strong that bad people have completely shut down if the Ghost Rider only glances at them.
The Ghost Rider is likewise virtually invulnerable, save from its enormous strength. While its flame skeleton can be technically damaged by sufficient effort, the Spirit of Vengeance is so rapid to regenerative that it has little inconvenience to do with (literally) bone smashing movements. Even the Ghost Rider can shrink their strongest blows and absorb additional damage, seemingly not in touch with pain, entities like Hulks and Hyde have found.
Then the Hell Cycle of the Ghost Rider. While various mystical vehicles were evocated by Ghost Riders, practically all showed outrageously great strength. Not only is the Ghost Rider incredibly quick and skillful on the motorcycle, the bike can accomplish inconceivable things, including driving buildings straight up and cruising across the river. The bike can even travel independently from Ghost Rider to start multifaceted attacks on his enemies.
And how is Ghost Rider precisely weak considering all this? Well, he isn't, he isn't... But a lot stronger he could be. He was a demon feared among other demons before Zarathos was linked to Johnny Blaze. He eventually became worshiped in exchange for an unchanging diet of souls by a native American tribe.
Mephisto actually grabbed the essence of Zarathos largely because he felt that the demon was taking away from the Hell Lords their fair portion of souls. While Mephisto was able to beat Zarathos, just how powerful it once was is that the demon was able to fight the Lord of Hell.
Indeed, Zarathos is seen to get stronger when he consumes more souls - and given how many souls he ate over his day, he can't really measure his real power. He can conjure storms, shoot flashes of lightning and even create earthquakes in whime. His physical strength far exceeds anything he shows as a ghost rider. He is immortal and incapable, even for the most powerful of cosmic entities, to be killed or injured.
Mephisto would not, of course, wish to see that his prisoner would have complete power - when he was tied to mortals such as Johnny Blaze, the mortal acted as a restriction to the power of Zarathos, enabling him to rip down people's souls on a far smaller scale. However, Zarathos sometimes exercises greater control over Johnny Blaze and makes the Ghost Rider more vicious and stronger.
Dr. Strange said at one point that the Ghost Rider might be in a power level like the "Green Scar" form of Hulk — a Hulk capable of starting earthquakes by strolling.
Fortunately, thanks in large part to the soothing effects of its human host, Ghost Rider rarely abuses his power.
It may sounds odd, but the spirit of vengeance can hold its full potential from being unleashed for all the heroic deeds Johnny Storm did as the Ghost Rider.