Last year I ran through LMOP with 3 bards(whispers, lore, and Valor) and an oath of ancients paladin. It was so difficult to challenge my players when 3 of them roll well on initiative and use shatter to end 3 enemies before the enemies even move.
Also throwing a magic immune enemy at a party of bards at level 1 is a total dick DM move
Goes both ways though, it is also IMHO a dick move to have your party shoot its entire load in the first battle then cry ceaselessly when you do not let them immediately get a long rest and dare to attack them again.
Heck, you blast off three Shatter spells and EVERYONE knows about you.
That's just one example. Even at level 1 having 6 spell slots for things like sleep or tashas hideous laughter or dissonant whispers makes challenging a party of Bards very difficult. For instance in Cragmaw cave in Lost mines the chamber where you can rescue Sildar 2 of them cast sleep to end the entire encounter. 1 dissonant whispers ended up killing Klarg because they all got in melee range with him. It was trivially easy
Have a monster or two make decent saving throw rolls and suddenly the party is screwed. You can potentially nuke foes but a couple good saves and you have nothing left.
A lot of beginning monsters have negative saving throws for the mental stats, so that makes that unlikely. Also things like sleep have no saving throw
First level spell DC will be 13 assuming point array. Assume on average 0 to -1 saves and you still have 30% of mobs making saves.