Iif you face reliability problems and change many parts early in the season, that triggers a snowball effect and you end up with poor reliability or big reliability risks late in the season, even if you improve at the factory... unless you produce 100 PU and change PU every weekend (similar to what Honda did).
Merc who did everything right did not have that problem and their PU, even old, were in optimal conditions.
That was one of Renault's problems last year... The causes and the consequences were:
-The ERS development and testing was rushed a bit
-dyno/track correlation wasn't good enough
-some supplier problems, some parts of the MGUH/turbo were not good enough
-early reliability problems in the season
-cooling problems for Renault and STR at-least
-integration was perfectible regarding some parts (hydraulic/water pumps for the RB teams)
-lack of spare parts at the end
-ending up with Frankenstein engines
...