I saw it when it was first on. I thought it was ok and kind of a strange letdown way of wrapping up the series, but I did NOT anticipate the level of vitriol that this particular episode would inspire. It’s not even close to being among the worst episodes of the series.
On a technical level, I suspect A Night in Sickbay would be the undisputed gold medalist of the ENT Bad Episode Olympics if not for TATV, with "Precious Cargo" as a narrow second (even Brannon Braga was embarrassed to put his name on that script!).
But as someone who was there at the time, there was just....a lot of things going on that contributed to that. Much like how Discovery is treated today, ENT had been slagged off as "fake Trek" for its entire run, with people dedicated to pissing about it every single week it aired. We were not at all quiet about how glad we all were for Coto overseeing S4 to the degree that would put the current Matalas love-in to shame.
Then the series was cancelled. Ratings were poor but from all accounts Les Moonves hated Trek and wanted to be rid of it and given UPN's network struggles, there was no way it would've lasted for a season six after the WB merger anyway. But we still started a fundraiser anyway to no avail.
And then the spoilers hit and good lord. Braga (who to his credit has since recanted because he's not entirely bankrupt creatively, unlike Berman) called it a "Valentine to the fans."
Which fans were those? The TNG fanbase who largely didn't fuck with the other shows (especially back then when we were still hating VOY pretty loudly and DS9 appreciation was muted at best)? The DS9 fans who barely got any acknowledgement even when the show was airing? The VOY fans who were still pissed about their lackluster finale that among other things stuck a last minute pairing for no other reason than a misogynistic dare?
It was certainly not for us, the underdogs who were all largely new to the franchise and who stuck by this show through thick and (very, very) thin, who loved the characters and who wanted to see THEIR story concluded. Not for yet another hour of TNG. Even the grew among us on the board who had been TNG fans saw this idea and went "bruh."
It wasn't enough to kill Trek entirely for me, but it did for a few others. I don't know if any of them have come back since Bermaga are no longer involved in any capacity but it's not for good reason Paramount pushed out "The Good That Men Do" as quickly as they did and made the rare call to declare it canon, which to my knowledge no other Trek book had been given before or since.
But the damage was very much done for many. We had a recent thread asking what happened to the OG fans and while I think ENT's cancellation between the transition from web 1.0 to 2.0 did a big number on that, this finale did it no favors to make fans WANT to add more fan media the way even the equally reviled VOY has gotten over the years.
TLDR: the hate for this episode snowballed for hella reasons, many of which had little to do with the finale itself and also possibly killed the existing fanbase at the time