It was a huge missed opportunity by not being made up of Nintendo locales full of Nintendo enemies and bosses. Yes there were some bits of this (goombas, skyworld, Porky, etc), but the vast majority was just generic areas with generic enemies. On top of that level design was pretty mixed, sometimes it was good but other times it was just meh, and I felt like it relied too heavily on combat arenas. I know smash is a fighting game but eventually I get tired of fighting big groups of enemies. Boss battles were good though.
I can also relate to your friends about the unlocking. Doing the SSE unlocks almost the entire roster for you without the excitement of getting that "Challenger Approaching!" screen and having to fight for the right to play as them. You could say "well just don't do it!" but I was like 13 when Brawl released, how was I supposed to know? By the time I was done it was too late and it kind of killed my motivation to engage with the rest of the single player content.
And coming off Melee, I found it kind of disappointing that the SSE was a one and done affair, unlike the Adventure Mode which was highly replayable. I just prefer having shorter stuff I can do over and over with the whole roster for fighting games.
That kind of sums up my feelings for Brawl. Even back when I first played it when I was 15 it was disappointing compared to Melee. I unlocked every single character in a single day with SSE, whereas I recall the struggle that was getting Mewtwo in Melee.