A full set of elastic and electrical coefficients of (K,Na)NbO3-based lead-free piezoceramics with a nominal composition of Li0.03(K0.48Na0.52)0.97(Nb0.8Ta0.2)O3 (abbreviated as KNNLT) was evaluated by the standard resonance method, and compared to those of K4CuNb8O23 doped (K0.45Na0.55)NbO3 (KNN-KCN) as well as typical Pb(Zr,Ti)O3-based piezoceramics PZT5A. The measurement of intermediate elastic stiffness and compliance coefficients of KNNLT indicated that KNNLT is elastically “softer” than KNN-KCN but “harder” than PZT5A. An extremely high piezoelectric stiffness coefficient h33 = 68.8 × 108 V/m was obtained, due to an especially low clamped dielectric constant ε33S of 361. And a large longitudinal electromechanical coupling factor k33 = 57% was observed in the KNNLT system. Furthermore, the piezoelectric coefficient d33 of KNNLT was characterized via three distinctive methods: the resonance method, the Berlincourt method, and the converse piezoelectric strain measurement; while these three techniques yielded values of 174 pC/N, 195 pC/N, and 308 pm/V, respectively. The difference in d33 values determined by the three methods may be attributed to different measurement frequencies and extrinsic piezoelectric contributions related to domain wall vibration and domain switching. Besides, the piezoelectric performances of KNNLT piezoceramics were found insensitive to frequency but susceptible to temperature and electric field cycling.