I often switch between several programing languages, I often switch between projects, I like being able to dig into the code source of some tarball I found online without configuration. I find that deep editor integration gets in the way more often than not.
For navigating the project I use helm on emacs, fzf on vim. I can just fuzzy-match anything I'm looking for instantly. I find filesystem treeviews useless, they use a lot of screen real estate and they're annoying to navigate, I don't see the point.
For navigation ripgrep does most of the job complex language engines do and it works across languages without any configuration. For C I sometimes use cscope and ctags but even there I often just call ripgrep because why not?
For auto-completion I also find that a very naive algorithm completing words from existing buffers works reliably and efficiently, with zero configuration and tweaking. Sure you don't get context-aware completion but that never bothered me too much to be honest.
For debugging I can't really disagree with you, both Vim and Emacs are very limited in that regard (Vim especially so).
That being said, maybe if I gave those big IDEs a chance I'd never go back, who knows. Besides I could spend an hour listing things I think Vim and Emacs do wrong so it's not like I think they're perfect editors.