1. extremely low filtration removal efficiency because the contents of the tank are very slow to 'dilute' by the return of such a small volume of 'filtered fuel oil', Most 'return lines' on small diesel marine engines are only a relatively few few cc's per minute flow.
2. causes the primary/second filter set to become more rapidly plugged thus less prone to capture 'slugs' of particles (eg.: particle storms generated from loosening of adhering tank wall sludge, etc.)
3. The more a filter becomes 'plugged' with debris the higher the differential pressure across the filter is needed to deliver the constant flow demand of the engine .... thus makes breakage of the lift pump diaphragm a higher potential.
The accepted way to 'polish' is to use a totally independent filter pump system, that has high volume turnover, uses 'more open' µM filter ratings ... more 'open' filters also means 'very cheap' filters.
Its the fast 'dilution rate' of particles thats occurring IN the tank by 'high turnover' or rapid high volume recirculation of fluid' over and over and over again through relatively low efficiency filters that makes the 'mathematics of reduction' the key to good and very economical 'polishing'; you simply cant get this volume flow from a 'lift pump'. There is a very significant (mathematical, efficiency, cost) difference between 'recirculation polishing' and 'single pass in-line filtration'.