I used GitHub for years in the tech industry, then went into the games industry and used BitBucket and hated it, thought it was such a downgrade. Now I'm back in the tech industry and using GitHub, and I miss BitBucket.
I liked the UI of BitBucket more. Stuff I accessed frequently like commits and branches were tabs across the top, easy to reach from any page. Branch dropdown sort order was by most recently updated unlike GitHub where I have to search for it. Easy to diff files. This was like 4 or 5 years ago though, maybe it has gone through modernization/enshittification. GitHub feels a bit fragmented and it tries to be more performant by virtualizing some things but while BitBucket in some sense was more rudimentary and showed it all (bogging my machine down some), it allowed me to CTRL + F easily with more confidence whereas with virtualization I've had issues with it finding things and I couldn't 100% trust it.
But CSS has bit me with heavy pages (causing a few seconds of lag that even devtools debugging/logging didn't point towards). We know wildcard selectors can impact performance, but in my case there were many open ended selectors like `:not(.what) .ever` where the `:not()` not being attached to anything made it act like a wildcard with conditions. Using `:has()` will do the same with additional overhead. Safari was the worst at handling large pages and these types of selectors and I noticed more sluggishness 2-3 years ago.