Well exactly, he's steadily been improving in every regard, but he's never had an end product issue. That critisicm is much fairer on a player like Coutinho or in the past Firmino.
Mane has outscored Firmino every season apart from last season, and has only 10 less goals in total (so far) despite being at Liverpool one season less.
As for his goal tally vs Coutinho, he's currently one goal behind him, after less than three seasons vs six for Coutinho. Admittedly Coutinho did play in a slightly different position, but Mane's output vs both players is streets ahead.
He's been a ridiculously good signing and is a better player than Firmino or Coutinho.