Let´s look at this objectively. From a 8.0-8.1 vs. 9.0-9.1 perspective.
Leveling:
BFA had six… SIIIXX leveling zones. You could also tackle the leveling zones in any order you pleased. Allowing you to skip parts you did not like.
Shadowlands only has four. Not only this, but the zones in Shadowlands are aesthetically poorer than the BFA zones, they are also a lot less fun. With players being forced to listen to exposition for 25 % of the leveling experience.
Winner: BFA
World Quests:
We don´t even have to debate this. WQ´s have never been worse than they are currently in Shadowlands. BFA already had worse WQ´s than legion, but there is no denying they are better than the WQ´s in Shadowlands.
Winner: BFA
Dungeons:
BFA had ten dungeons on release. Shadowlands only eight. I would also argue that dungeons like Freehold and Atal were superior to any of the Shadowlands dungeons. Running the same eight m+ dungeons a year into the release of the expansion feels awful.
Winner: BFA
Raiding:
Now this one is actually close. Castle Nathria is superior to Uldir, no questions asked, even with how buggy and unbalanced it was at release.
Battle for dazar’alor was one of the best raids Blizzard has ever released, and was actually decently balanced upon release. Gameplaywise, loot-wise and aesthetically superior to Sanctum.
Uldir does, however really pull down the raiding in BFA.
Winner: Very small win to Shadowlands
Systems introduced:
Ah yes, the ultimate battle. Azerite armor versus Covenants + The Maw + Boreghast? What system is worse. People are going to complain about grinding azerite, but the truth is this was only a problem and felt bad within the first 6 weeks of 8.0 and 8.1. With the catchup mechanisms in place every week, grinding azerite was a joke after this, and barely something you actively had to do on new alts created two months into the patch.
Azerite armor was awful, and didn´t get much better until 8.1 where they released the m+ vendor that allowed to buy pieces. Luckily, 8.1 was released pretty quick.
Covnenants however? Being forced to complete the story on every single character for an entire year after release is just insanity. Being locked out of all these cool class abilities, because of choices needing to matter or whatever. Unforgiveable.
Whilst azerite armor was awful, I would argue that covenants and the covenant leveling system are much worse. AP could be grinded casually very soon into patches, and you never had to step into a single island expedition if you weren´t a mythic raider.
Now if you compare Boreghast and Island expiditions? You actually never had to complete Island expiditions, whereas you have to do Boreghast to unlock legendaries. You also have to do Boreghast if you want to UPGRADE said legendaries.
Winner: There is no winner, both systems were awful and we are all losers here. Shadowlands infinitely worse, unless you were a mythic raider for the first weeks of release.
Alt friendlyness:
In BFA by the time 8.1 released all you had to do was level a character to max level, passively get AP from things that upgraded your gear, like WQs, warfronts and M+ and you were ready to raid, m+ and pvp. If you played 6 weeks after the release of 8.1 when AP catchup was strong, this might have been the most alt-friendly time to play WoW since the release of Legion. You could also get good gear really fast if you wanted by just grinding M+ and raid on your new alt (with bonus rolls). Whereas now with loot scarcity, you are forced to wait for your weekly vault to slowly upgrade your gear every week.
Shadowlands: Having to finish your covenant story + Having to do Boreghast + Having to grind your specific class legendary (I´m going to ignore the awful socket grind, since it is very min-max, it is however another awful Shadowlands system both in 9.0 in The Maw and 9.1 in Korthia).
There really is no competition here. BFA was in 8.1 at least, and 8.0 infinitely more alt friendly than Shadowlands.
Winner: BFA
Conclusion
BFA was on almost every objective metric, a superior expansion to Shadowlands. People who are talking about rose-tinted googles, about an expansion we didn´t even like, and that was out 2-3 years ago need to stop. Nostalgia isn´t why BFA was better, BFA was better because Shadowlands was simply worse.
(Note: I am only comparing the release and first patch of both).
9.2 was an atrocity, and makes BFA look even better!!! Shadowlands not even having a 9.3 does not help either.