In a high-level campaign, with access to lots of expansion books, bards can be an effective class. (If nothing else, they are a spellcasting class, and spellcasting classes become very powerful at high levels.)
However, when a player is new to the game, if they build a low-level bard character it is likely to be a flat-out non-combat character. (Reading the bard character description, they can get the impression that a bard's role in combat is to spend the whole time singing Inspire Courage while the rest of the party does the actual fighting.)
This might be a good fit for some campaigns (those that contain no combat or very little combat) but in other campaigns it can be unplayably bad. The character might get less unplayable at higher levels, once it starts getting good spells, but many players will get frustrated and quit the game before that happens. I have practical experience of this.
The stereotype of bards being weak probably comes from observation of low-level high-Charisma bards in combat-centric adventures.
Let me offer a specific example.
I had a player who played a paladin character; it was an effective and useful character and she had fun in my game. In the next campaign she played a druid; it was an effective and useful character and she had fun in my game. In the campaign after that she played a bard.
We started at third-level, as is usual. We got into combat and she didn't have anything to do; she didn't have any useful spells, she didn't have a weapon, and she hadn't thought that would be a problem for a Charisma-based character focused on the Diplomacy skill. She spent the whole time hiding behind the fighter. After two sessions she stopped showing up and I never heard from her again.
When I say "the bard class is too weak", this sort of experience is what I am talking about.