Meh Mongose cores are the worst arm design ever made & M5 leads the pact. Worse part is they cut caches size for mid cluster (A76 core's in this case) & little A55 one's. On the other hand QC did unexpectedly well with A77 core's integration so much that they are more power efficiency than A76 on any SoC & significantly improved cache speed and coherence (which whose really bad on couple previous designs), this helped both with performance and efficiency. There's also a problem with Mali G77 on Exynos so either Samsung did something wrong or ARM lied. They both throttle almost equally with GFX while Exynos in this case utilise A76 core's, never the less old Adreno is still significantly more power efficient while it's ALU performance (3DMark) is expectedly unmatched. All this while Exynos 990 is based on still more advanced manufacturing process. That's a short version for detailed more comprehensive one head to AnandTech.
Of course Samsung will never publicly acknowledge this but they already publicly anonced decisions that they are giving up on future development of costume (Mongose) CPU core's really tells it all.