And what happens when a billion people who claim to be "indwelt by the Holy Spirit" all interpret the Bible differently? From Benny Hinn to Harold Camping, they all have a different understanding. Just sayin'.
Actually, even having a supreme authority will not prevent the need for interpretations and people having different interpretations, and lack of complete unity. But unity in core teachings at least can be realized, as well as limitations in disagreements.
RCs must rely on fallible human reasoning in deciding to submit to Rome and then must discern which magisterial level as teaching falls under, and thus what degree of assent is required and if any dissent is allowed, as well as what they all mean.
RCs must give complete assent to certain core teachings, yet while Catholic teaching limits disagreements, the things RCs can disagree on is substantial .
And then there is the issue of what Rome effectually teaches, which interprets what she officially states, and results in an even greater variety of Catholics.
Moreover, the RC has great liberty to interpret the Bible to support Rome, as only a few interpretations will be excluded with certainty by what any of the four factors (passages of Scripture, by the judgment of the magisterium, by the Church Fathers, or by the analogy of faith) are understood as teaching.
Rather than any kind of comprehensive doctrinal unity, under the Roman model for unity, in which the church is supreme (sola ecclesia) you have sects within Rome as well as divisions without, both disagreeing on what Catholic Tradition, Scripture and history teaches, each infallibly interpreting it to support their side. . And which model is also shared with cults such as the LDS.
On the other side, while there are certainly and sadly divisions and disagreement, yet rather than doctrinal anarchy, those who take the Scriptures to be the supreme authority as the literal Word of God consistent with the evangelical "tradition," have overall historically evidenced a shared assent to salvific core truths, and which was manifested in a common front against cults which deny them, as well as liberal revisionism of Scripture (thus the rise of the modern evangelical movement 100 years ago), and which Rome herself is guilty.
And while both faiths are seeing declension is the latter days,. yet evangelicals testify to greater unity in moral views and certain truths than Catholics overall, the majority of which are liberal, yet whom Rome typically treats as members in like and in death.