So, as has been said, it caused imbalance because while the PCs got all these advantages, the monsters didn't. And again, pre internet days, so we didn't have smeg-tons of materials to work from. The other option was to build your own monster versions or upgrade the monsters. Frankly, it was a lot of work for the DM to do all that. And to try to keep it fair.
The good ideas, though, including some spells, weapons, and classes, survived into 2nd Edition. The bad ideas... didn't.
And then the system got redefined and cleaned up into 3.0, refined into 3.5... 3rd at least had built in tools for using a computer to do character creation.
And when 4th came out, the Edition Wars began...
3e and especially 3.5 would be your bane if you didn't like to build your own monster versions or upgrade monsters.
They almost demanded you do it for every monster encounter the DM made, and was DM intensive in monster handling and creation.
Far more, several times more, than ua or anything in TSR D&D. If one loved 3e and 3.5 I don't think they have anyplace to complain about that (or much of the UA either, considering how much 3.X powered up spellcasters and some other classes, heck, you were not even assured of disrupting a spell anymore even if you smashed the caster hard!).
As far as those angry between the changes from 3e and 4e...that's because they don't recall how upset many were for WoTC killing off D&D and replacing it with their own game under the same name and moniker. Far bigger and far worse than any "edition war" that came after and the casualties were far greater. WotC lost over 20 million fans that day (though most had left already having other things take up their time, most still looked on TSR D&D fondly, even if they were not actively buying anything). D&D went down to less than 6 million players, and never recovered from that until a decade and a half later (over 15 years before they got over 20 million 5e fans, aka WotC D&D fans). Nothing compares to how badly they lost fans (they gained buyers, but not the old fans) and never recovered from that for over a decade. 4e only lost like 2 million fans (though also gained buyers at first, though that decreased...and I suppose 2 million would be a little less than half of the players, so percentage wise it may have seemed a large thing to you). With the change over from 3.X to 4e, those that were just mainly 3.X fans finally got to experience what the rest of us suffered (all 25 million of us, most who left D&D and never came back to it...farewell my friends who left in disgust) because they did the same thing again (made a new game and tossed out the old, calling the new game D&D instead).
5e did the same thing, but enough were now accustomed to this changing the game entirely with new rules as an edition change that it didn't have as much upset people and fans.
However, with how much 5e has grown, if they try the same thing again with 6e with a radical rule change...I expect the uproar to be louder than it was between TSR and WotC D&D (which by default was FAR worse than it was between 3.X and 4e).
