If ideology were nothing other than “adherence to a coherent set of beliefs,” then there is nothing wrong with it. But that is not all it means.
An ideology is in some way disconnected from reality. A coherent set of beliefs becomes ideological as soon as it is no longer tested against the concrete facts on the ground and adjusted accordingly.
Ideologies are not just partial and incomplete, like any world-view, but closed off to genuine dialogue and to the correcting influence of stubborn facts.
If the facts don’t happen to fit the ideology, the facts are chopped off and discarded.
Like supply side economics, which just don't fucking work because....stubborn facts.