Ok. Let's go on the opposite direction. My biggest issue against multiclass is that it is very difficult to get it right. Most people that multiclass are looking for specific "builds", and so came the "dip" concept: maximize damage and DPR are usually the main subjects on multiclass. The concept behind the second class fails to keep true on these dips, and are usually for the forefront abilities. There are some classes more optimal than others for these: the star classes to dip ar rogue and bard, for the expertise (and perhaps the spells).
BUT the main problem with these is that aside for that classes there is hardly a reason to dip only one level.
1) There are stat prerequisites to multiclass. They aren't very hampering, but they exist, and they can divert a character from its focus.
2) Multiclass never grants the full spectre of proficiencies. Classes (such as barbarians) dipping on Fighter, for example, never gain Heavy Armor proficiency.
3) Falling behind. In theory, combos work swell, but in real gameplay, their real synergy quickly falls behind of a proper class.
Most builds are given with an objective in mind, such as having at 10th-level a certain amount of DPR via cantrips, or maximize certain effects, but in game they are delayed from powerful effects, having first tier abilities of their main classes for (at least) half of the second tier, and delay the 3rd-level boost (archetypes or powerful abilities such as 2nd level spells). One of the most praised combos, such Bardlock, needs at least 2 levels of warlock to have Invocations and 2 spells per short rest (without even hitting the Pact Boon), and 3 levels of bard to have an archetype; at fifth level you merely gain 2nd-level spells and only there gain expertise, whereas a single class bard has ASI, its Bardic Inspiration escalated to d8 and recharges on short rests, has 8 spells and 3 cantrips, and casts 3rd level spells such as Fear. At sixth level, while the Bardlock gains its first ASI, the Bard has another College feature, such as Extra Attack, Additional Magical Secrets, or Mantle of Majesty.
I have a sound example. My party has recently reached 6th level. As a human Battlemaster, I've reached 6th level with 3 feats: Prodigy, Alert and Shield Master (I had luck and rolled an 17 in stats, otherwise I would ditched Alert). And I'm a real beast: with +10 to Athletics, there is no troll, ogre or mount that could resist my shield shoves. I'm dishing 34+ damage each round, and knocking prone almost anyone with my shield (I'm mostly on horseback with a lance, thus also avoiding damage), I can't be surprised and I go first most of the time (+5 to Initiative). I have advantage on most of my attacks. Also, I have
a ton of utility. With a crowbar or ram, I act as a rogue of sorts of the party, forcing chests and knocking down doors. Also, I have 6 skills, 3 languages and 3 tools (smith's, tinker and carpenter's). I can track, run, mount, bend bars, break and climb better than most classes. I'm scary as




(Menacing Strike and Intimidation proficiency). And I can block bolts and such with my shield (I have 1 magic object, and is a +1 shield, -thus I have +3 to Dex saves-).
The bardlock in my party only recently had its expertise (one level before me), and he can't even cast 3rd level spells. Yes, he does 2d10+Cha with an Eldritch blast, and recharges healing spells on short rests, but that's all. The Tabaxi monk, also, can attack 3 times without expendig ki, moves 55 feet per round (Mobile feat+Monk speed), doesn't provoke OAs, and stuns up to 4 creatures per round, whenever he isn't controlling and moving the enemies in the battlefield. He can also climb, sneak, dodge and grab decently. Perhaps in 4 levels the bardlock catches up, but by then I will gather enemy info only by observing, I will have a 20 in strength, +13 to Athletics, reroll failed saves, 5d10 superiority dice and 7 maneuvers (and perhaps my flamethrower, with tinker tools and Alchemic fire).
I'm not saying that multiclassing is
bad per se. I'm saying that it is difficult to get it right and advance properly without hampering yourself too much. Going back At this time, if he were a pure Bard or warlock,
he could make my horse fly and I could rain death from the skies.