harden has had one season when he had TS% (TS takes account different shot shot types, 2pt, 3pt and FTs) that was below lavine's best, harden's rookie year. his 2nd worst TS is better than lavine's best. his rookie years TS also beats lavine's 2nd best. harden's rookie year is the only year when he had below average PER. lavine has had that every year so far on his career. harden is better FT shooter, gets more FTs, shoots relatively more 3s compared to 2st than lavine and shoots 2s more efficiently. hardens teams also do better job at outscoring the opposing team when he's on the court vs when he's off. the opposite happens with lavine.
the post you quoted literally had another example in devin booker who many people aren't sold on despite scoring more and with better efficiency than lavine. it's not lavine vs. mcdavid, it's hockey vs basketball. lots of players in the past have put up points with bad efficiency on bad teams. exactly/about the same mins (27/g) and points (~17) as lavine is more rare. booker would be a lot more apt comp for mcdavid and he doesn't even come close.
i do agree he has more potential than what he's shown so far but not on skyhigh. not being great natural shooter and BB IQ issues put a limit on his potential.
it's not just on these forums were people are low on lavine and/or think this contract isn't good. check any major NBA forum and you have people saying the same thing.