Drawbacks in Twitter Bootstrap

From last year onwards, when I first saw and used bootstrap, I was a great fan of that. I used it in one of my ASP.NET MVC project and I was actually in love with it. It makes life so much easy for the developers that is not very good with front-end development (as in my case :-D ) as well as for rapid application development and prototyping.

Bootstrap, a sleek, intuitive, and powerful mobile first front-end framework for faster and easier web development.

Quote : http://getbootstrap.com 

But, recently I have heard some negatives about Bootstrap framework from few of my UI expert friends. I tried to list the drawback points below.

• Adds more than necessary (bulky) HTML and CSS to a web page
• Hard to structure/design web pages as necessary
• Contains some accessibility issues - Ex: color contrast issues, non-sementic markup, etc
• Released versions are not backward compatible
• W3C HTML and CSS validation issues

Alternative/Options :

• Customize the bootstrap framework to fix these issues
• Foundation 5 framework - http://foundation.zurb.com     
• Note: I have not used this. One of my UI expert friend highly recommend this framework

Some good links regarding this issues :

http://www.wearecube.ch/limitations-of-twitter-bootstrap/
http://stackoverflow.com/questions/14227000/pros-cons-of-switching-to-twitter-bootstrap
https://github.com/dobtco/bootstrap-accessible/issues