yeah, he really dropped the ball to share his opinion without a close look at the actual math of what AC different classes usually have, what HP, and what damage. while he can have his opinion, its bad for him that he did not use good arguments for his rant.
lets look at the facts then. point buy, my standard will be an open-hand monk, versatile human with str 10, dex 16, con 14, int 8, wis 16, cha 8. this is the 'default' race and subclass, and the best way to use point buy for the class. the monk uses a versatile spear for 1d8+dex until level 11, when it no longer matters what weapon he uses.
martial classes are Fighter(d10), barbarian(d12), Paladin(d10), Ranger(d10), Rogue(d8), and Monk(d8).
point buy at level 1, monks can have AC 15 or 16 depending on race. Rogues get 11+dex for 13 or 14. so the rogue starts with the worst hp/AC hands down. but the fighter may have an ac of 14, if hes an archer with bow and leather armor instead of the clasic chainmail+shield AC 19 tank (arcane archer is a cool choice). the barbarian starts with no armor, so 16 AC max until he buys something, and probably only a 15 because str+con is more important than dex+con. the ranger has scale (14+2) or leather (11+3) for 14 to 17 armor. the paladin has 16 to 19 for AC.
so overall, monk AC is really good at level 1. the only ones with better AC have a fighting style and/or shield. and monk hp are the same as the rogue, but with more AC. monk AC progression is tied directly to stat increase, and should hit 20 at level 16 before magic items. the rogue should be 18 before magic items, so in a low-treasure game the monk wins easily. even a fighter or paladin needs a shield to match a monk with no magic armor. the barbarian can hit 22 at level 20, with 24 con and 20 dex, but thats a capstone bonus.
stunning strike con saves... yeah, its an issue for most 'large' monsters like giants and dragons. those tend to have a lot of con. but how often do you reall fight those? stun works on ALL enemies, lasts more than a round, and nothing is immune to it. you will go from a DC 14 at 5th level to a DC 19 at 17th level (DC 21 with both tome and a blessing). a CR 5 hill giant has a +4 con bonus for your DC 14, so thats a 50/50 chance. a CR 17 adult red dragon has a +13 con bonus and 3/day legendary resistance so you are not likely to stun him with your DC 19, thats a 1 in 4 chance. AC 19, your base +11 to hit means 3 out of 4 attacks should hit on average, so 3 chance to stun per round for 4 ki. over 5 rounds, thats 15 chances to stun before your ki is gone (and some will be spent to reroll saves), so you can probably blow through his legendary resistances to stun him once if the fight goes on long enough. and he ALSO has to make saves against flurry of blows each round (only +6 dex), which increases the chance of blowing through his 3/day.
damage? oh, more than a fighter with a 2-handed weapon for levels 1-10. fighters pull ahead at level 11. extra damage is spread across more rounds however. a monk does 1d8+dex and 1d4+dex at level one (avg 13), and 1d8+dex, 1d8+dex, and 1d6+dex at level 5(avg 24.5). the fighter does 2d6+str at 1st (avg 10) and twice as much at 5th (avg 22). also, at level 6 the monks unarmed 1d6+dex strikes are magic weapons for bypassing resistance, so this is great in a low magic campaign. and the rogue is doing 1d8+1d6+dex at 1st (avg 11), to 1d8+3d6+dex at 5th (avg 19). yeah, rogues are the low damage, low AC class. it just looks more impressive as burst damage in one attack. the monk even hits more on average through 10th, because he has more attacks. campaign stops before high level? guess what, the monk did more damage than the guy with the 2-handed sword on average.