I've been using XHTML for about 3 years now, and didn't care much about HTML5 and CSS3 arrival. Several months ago I've stumbled upon HTML5 Boilerplate and I'm starting to get very interested in it right now.
The only thing I can't find yet is drawbacks of using this set of best practices. I know, that the main goal of it to make site look the same on wide variety of browsers, but is there something I should be aware about?
Is there something I should be aware about?
Yes, definitely. It's not a plug-and-play thing. Or at least, it could be like that but it hasn't been conceived for that. And if you use it this way you can't enjoy its best. Boilerplate is very flexible and you have to learn how to customize it. Full stop.
My personal suggestion is to start experimenting with it and study the features it has to offer. Luckily for us it's well documented and you can learn a lot. Not only the boilerplate itself, but also the rules and best practices about HTML5, CSS3, resources loading and related issues/workarounds on performances, browsers quirks and how to fix them, tricks for mobile development, polyfills and conditional loading and a lot of other crazy stuffs. Once you are aware of its possibilities, if you'll find drawbacks you'll be able to customize the base boilerplate.
Start from here, read the docs, follow every link and don't be tempted to take shortcuts. It takes time, but you'll be rewarder very well. If you have troubles ask.
For something more concrete, a drawback I found very quickly (but someone consider this a plus) is that the boilerplate doesn't provide a CSS grid. Not a huge problem, I discovered that adapt.js is easily integrable, so I replaced normalize.css with adapt.js's reset stylesheet and used the grid.