I agree containers are confusing but honestly making a custom web app look good on all resolutions/aspect ratios is complicated in my opinion.
However, if you know how to work with Bootstrap, I don't see a reason to learn something else for side projects, if you want to minimize the effort you put into front end stuff. It might not be ideal, but it's good enough imo.
That said, the Bootstrap 4 defaults are much more modern looking than Bootstrap 3's.
Note the criterion here is speed, not modernity or aesthetics.
Edit: by speed, I mean speed of development, or ease of development.
I've found that bootstrap remains the best option to create a reasonably responsive, simple layout that is still quite usable on mobile as much as on the desktop, with no more than an hours' worth of work. And you don't even need to know css really.
I've used vanilla CSS and HTML and even after hours of amateur effort the site always looks like it came from the 90s. I don't have the time to worry about what font family I want to use, is that too much to ask?