All I know is, I replayed the harpy fight recently, and it went ridiculously bad because they kept flying higher and getting height advantage and beating the crap out of my party.
Then,, while writing the Afflicted story, I played with a party of 6 level 3 characters, Gale, Wyll, and 4 Custom using Tabletop, 5e rules, etc. And legit harpy stats. It was not nearly as difficult, BUT they nearly took out my female Drow Battlemaster and my Beastmaster Gith and Gale. It was still a fairly tough battle that could have gone south with better rolls on my part for the harpies. The only variant rule used was flanking.
The point is, 6 party members. No height advantage. No homebrew. Normal stats. 4 harpies. It was a fun battle and wasn't frustrating because the players could actually hit and weren't just getting the crap beat put of them by harpies with advantage.