Agree with rkieru. Bootstrap is never my first choice to use, and I have my own very simple/clean starter “framework” for my own projects where appropriate. But if a client requires it for reasons that are important to them, and if I want to take their work on, then why now?
And I’m actually in the middle of quite a large online app, and using Bootstrap for that has been a godsend in simplifying so many aspects of it! I can build the core structure and functionality quicker, with a plan to go back towards the end and do more style and visual customisation to remove most of the “Bootstrapness” look.