Global Navigation Satellite Systems (GNSS) are fundamental for positioning, navigation, and timing (PNT), playing a crucial role in next-generation intelligent transportation systems and safety-critical applications. However, achieving precise PNT solutions in challenging environments remains a significant challenge. Under ideal conditions, carrier-phase-based techniques such as Real-Time Kinematics (RTK) and Precise Point Positioning (PPP) enable high-precision positioning. However, their accuracy heavily depends on the quality of phase observables, which can be degraded in harsh environments, such as urban canyons or interference-prone scenarios. A promising alternative is the use of large-bandwidth signals, which enhance resolution and improve code-based observables. This can be achieved through high-order Binary Offset Carrier modulations or GNSS meta-signals. This study investigates the fundamental performance limits of time delay and Doppler estimation for such signals in challenging scenarios, particularly in the presence of multipath interference, where signal reflections significantly impact receiver performance. Characterizing multipath effects is critical for the next generation of PNT applications, as it directly influences the robustness of GNSS solutions. To analyze these effects, we derive the Cramér-Rao Lower Bound (CRB) for time-delay and Doppler estimation under a signal model where one specular multipath degrades GNSS receiver performance. This case considers that the receiver is aware of the multipath and applies countermeasures. In the second case, we assume that the receiver is unaware of the multipath, for which we derive the Misspecified CRB (MCRB). The MCRB quantifies the performance degradation in standard GNSS receivers due to unmodeled multipath interference. We validate these theoretical bounds by comparing them with state-of-the-art estimation algorithms. Our results demonstrate the significant performance improvements achievable in harsh conditions using metasignals such as Galileo E5a + E5b or GPS L2 CM + L5, compared to legacy signals such as GPS L1 C / A.