Not every link helps, and some actively hurt. If you have inherited a backlink profile you did not build, or you are evaluating a provider, here is what to look at.
Over-optimised anchor text
A natural profile is dominated by branded and generic anchors. When a large share of your links use exact-match commercial anchors, that pattern is detectable and it is one of the more reliable ways to attract an algorithmic problem. Aim for the majority to be branded or generic.
Private blog networks
PBNs are networks of sites built to sell links. They share hosting patterns, ownership fingerprints and outbound link behaviour that make them identifiable at scale. They can work for a while. When they stop working, they stop working all at once, and the cleanup costs more than doing it properly would have.
Irrelevant placements
A link from a site with no topical connection to yours contributes little and, in volume, looks like exactly what it is.
Sudden velocity changes
A site that has earned five links a month for two years, then earns four hundred in one month, has visibly done something. Growth should look like growth.
Auditing an inherited profile
Export your referring domains, sort by traffic, and look at what is sending none. Check anchor distribution. Look for footprints across domains — the same template, the same hosting, the same outbound patterns. Disavow is a last resort, not a first move; most profiles are better served by building genuine links to dilute the bad ones than by mass disavowal.