I have nothing of value to say about bearings. i know ceramic ones make no sense for cycling. I know Hambini feels strongly about them. I know my ZTTO BB spins so well without load you'd almost want to post it on insta. Beyond that...
I am very pleased with my er9. When it's charged (it doesnt work if it's not charged, and that is not trivial, i am an imbecile after all, the thing died on me twice already because i thought it was charged, and wasn't (my fault entirely)), it's really great. Now, assuming you maintain your drivetrain and set it up properly, mechanical works super well. I think the biggest difference is front derailleur shifting. One cute trick i found with electronic is shifting the front & rear up or down at the same time. It's flawless and makes shifting the front smoother than usual. Doesn't work if you're already at the end of the cassette, but it's not something i would ever do on mechanical. I also find myself shifting much more on electronic because it's effortless and instant. Front shifting on mechanical does require quite a hand movement, electronic is just a click.
So, do you need it? no. Is it nice? yes. For c.400 usd (if not less) for the er9, i think it's super compelling. I also like the gear protection option (you can't cross chain the last 2 cogs), and, super important, you can change from 7 to 12s in app. The last batch of 4 bikes i built, i bought a mix of 11 and 12 components for various reasons, it's incredibly helpful to install whatever on the bike and just change it in the app (I'm thinking large cassettes especially that exist in 11 but not 12s, for example, like 11-36 superlight).
Now, if the groups start dying on us, i'll do a 180 degree on my opinion, but for now, we all love our groups. I have an older friend who's got several s works, ultegra & dura ace (not the latest). He says the er9 is faster, smoother and more silent than his ultegra. Having ridden behind guys on ulegra di2 (not the latest), Ultegra has a distinct sound which the er9 barely has, it at all. Arguably you can sneak out on friends that way, like if you shift intelligently, you can outsprint them and they wouldnt hear it coming.
I havent ridden in the rain, i know some people have reported problems. That does worry me. I bought liquid eletric tape but i need to 3d print some narrow applicator cap thing because i currently have zero precision to seal the connector on the RD. Afaik, people who sealed that port have fixed any problems they had with waterproofing. At which point, i think the thing just works.
But again, only time will tell. So far so good, but we've ridden probably 2-2.5k km total, which really isnt much.
Last, my bias. I think most of SRAM innovations in recent years are an insult to engineering, and i think most of what Shimano's been doing in years is patent trolling. So i vote with my money as well. I'm a contrarian, and i have little interest in funding the marketing budgets of big brands. As far as I'm concerned, Jumbo visma can buy their own groups, just like i'm not paying for the shoes that OM or PSG football players are using.
Rant over.
