TW dungeons have different mob/boss scaling and difficulties based on expac. i noticed it most on my 20s after DF prepatch.
BFA dungeons unless something has changed are the most difficult, ironicly that's where new to WOW players are placed by Blizzard. another thing you have to consider is mixed level players with higher level abilities/talents
dungeon/questing player experience and learning classes was lost over a decade ago. WOD has been well talked about for years even after objective XP was cut, it's the fastest expacs to level. it's the main reason players go there, including new to WOW on their second players. that's why after having gone thru BFA leveling they are the ones predom saying "WTF is going on here?" in WOD dungeons. the classic, BC, WOTLK are also easy. Legion depending which one are slightly harder. SL dungeons involve more trash control, but still not as hard as BFA
the reason WOD dungeons are getting attention of twinks all levels is the WF gear and word is out that if you want fast leveling que WOD dungeons. during SL the word was out about twinks predom queing BC dungeons for socket gear.
abilities that some low level classes have at 10 is what is breaking. there are alot of 10 classes/specs you do not see doing the speed runs. the trinket exploit also enables the ability to blow thru for some low levels that without it could not be done. to get that their key class power ability you see arc mages at 11 (most stay 10), guardian at 11, fury wars at 14, etc.
today i was in a group with my 10 monk (made a few days back) just going thru, not blowing thru. not built for that and no trink. the healer got mad and stated, "why am i a healer, no one needs heals" and left after the first boss. we never got a healer rest of dungeon. at the end i looked at who we had and it made sense the three DPS were all in their 50s. DH, FDK, Fwar who all have alot of healing attacks, damage negation, more talents, passive leech etc.