Net thrust of the engine is always gross thrust minus ram drag. Take away the net thrust, and you're left with ram drag, also referenced as inlet drag or intake drag. Spool the engine to greater power settings, and the drag rise increases.
Whether a straight turbojet or a turbofan, the inlet drag is considerable; the engine produces much more thrust than what is usable to propel the aircraft, but a certain portion of that thrust is used to overcome the drag produced by the engine. Higher velocities and higher power settings produce more inlet drag.
When reverse thrust is used, the thrust produced by the engine (by fan or by jet exhuast, depending on the powerplant), is diverted. Generally not directly against the direction of travel. The contribution to slowing the aircraft during landing, by re-directed exhaust gasses or fan flow is relatively small. The redirected gas path is often only slightly forward or at right angles to the direction of travel, and doesn't produce significant retarding action or deceleration (acceleration, to be correct).
Some aircraft will produce enough reverse airflow to back the aircraft, some won't. In either case, the contribution of this airflow to stopping the aircraft during landing is relatively slight.
The drag rise in the engine, specifically at the inlet, accounts for most of the retarding action used during landing. The faster the airplane, and the higher the power setting (the faster the engine is spooled, the more thrust produced, but take away that thrust, and the more drag remains. It's this drag that accounts for the effectivity if reverse thrust during landing.
Re-ingestion affects engine longevity, but doesn't account for a change in effective reverse thrust during landing.