With the increasing use of Field-Programmable Gate Arrays (FPGAs) in critical applications, safeguarding against malicious attacks becomes necessary. ElectroMagnetic Fault Injection (EMFI) stands out as a potent threat among localized fault attacks with its optimal compromise between cost and effectiveness, without the risk of damaging the target chip. Among potential targets, Ring Oscillators (ROs) are critical components that can be used in secure primitives, as well as detectors against physical attacks. In this paper, we analyze how the use of FPGA resources affects the outcome of EMFI attacks: we experimentally show with single EM pulse injections on three families of Xilinx FPGAs manufactured in 28nm process technology that the harmonic response of a RO heavily depends on its layout and density within the FPGA die. We also highlight the need of considering both EM pulse polarities when evaluating the efficiency of any proposed countermeasures, as this can reveal different sensitive locations on the chip. These findings can be leveraged for designing architectures that address the EMFI threat more effectively.