What are the drawbacks of HTML5 presentation software?
From a web developer perspective, the only issue I can see is compatibility. If the provided computer does not provide an HTML5-ready browser, the presentation may not work. Unfortunately, having been in labs and lectures with institution-provided computers, there is no guarantee that an IT department has provided the newest browser. While I'd like to think that people, especially at institutions, are keeping their machines up to date, we have to assume that some machines have been forgotten or have not been maintained.

This HTML5 readiness graph demonstrates that IE8, which still has a huge market share, is woefully prepared for any HTML5 feature. IE9 has some functionality over IE8 and appears to cover the audio/video portions you require, but only IE10 appears to be HTML5-ready as of last year.

I tried the reveal.js demo on IE8 just now. The rotating transitions don't work at all (it acts like Powerpoint basically), but it was usable. Since there are no videos or audio, I can't test those, but a standard presentation would at least be doable even in that browser.

It is unlikely that any IE9- browsers will ever reach full HTML5 support due to the security risks and time drain required, so any attempt to present on an IE8 browser is likely to be bare-bones and equivalent to viewing a PowerPoint.

However, this can be largely mitigated with the use of a PortableApp web browser such as Portable Google Chrome. A small footprint that can be carried around in a flash drive with the presentation.

From a student perspective, reveal.js might appear to be more confusing if you share the files, as it probably isn't just a foo.ppt file, but a source document, the JS, and so on. You will probably need to provide a read-me for being able to view the presentation. I'm sure everyone here knows how to open and view a PowerPoint, but some of us might stumble a bit with a more complex solution that may depend on multiple files and folders. That, and the student would need an HTML5 browser to get all the features working.

You probably will also need to direct PDF copy of the slides because I didn't see any noticeable way to print the presentation unless the stylesheet natively does that from the print menu.