The intercropping of winter oilseed rape with frost-sensitive service plants can provide a diversity of services including weed control and N supply for oilseed rape. This practice started to be adopted by farmers and has therefore become one of the most popular intercropping in Western Europe. However, in Switzerland, such intercropping leads to contrasting yields. The growth of service plants and the benefits they provide have also been found to be variable. The factors explaining these variabilities remain unclear. Our study aimed to better understand this variability under a temperate climate thanks to the regional agronomic diagnosis framework. In this study, we first investigated the main factors explaining this variability and then aimed to rank them to identify ways to better manage such intercropping systems. A network of 28 farmers’ fields planted with winter oilseed rape intercropped with service plant mixtures was studied. Farmers’ practices were diverse in terms of specific composition of the service plant mixture, pest management, and fertilization. We observed that the growth of oilseed rape and service plants in fall was highly variable. We determined that in late fall, the main drivers of the service plant mixture dry weight were specific composition of the mixture and precipitation. The introduction of buckwheat in the service plant mixture enhanced its late fall biomass. The oilseed rape grain yields ranged from 0.4 to 5.0 t/ha and were lower than that of the local reference in 75% of the fields. This was mainly explained by insect pest damage in spring due to a very limited use of insecticide in our field network combined with a lack of alternative pest management strategies. This work provides further elements to investigate the causes leading to the high variability we observed, together with the local observations that will benefit the farmers.