Other than the examples you gave, there should be no other general liabilities. Do remember, tho that books over 20mb need to be downloaded via a docked or Wi-Fi connection. I don't think that is a deal breaker, but it is another data point in the topic at hand.
As always, run tests that allow you to judge the results as pertains to your content in particular and then decide. If you find bloating (max size is 2GB in any case), or performance issues, then you may have something more to consider.
"Are there disadvantages in making each page one single image over the full screen, and using text as image, rather than actual text?"
There are three disadvantages. Firstly, full page images will bloat your file size over the alternative, half-page image. Secondly, editing your text becomes a pain. Thirdly you won't be able to use the Glossary facility, although this is unlikely to be a big problem with that sort of book.
Firstly, full page images will bloat your file size over the alternative, half-page image.
Secondly, editing your text becomes a pain.
Thirdly you won't be able to use the Glossary facility.
1. Not true if designed and saved properly in a graphics program.
2. All text editing should be done in the graphics program, not iAuthor.
3. Not important.
Construct your pages in a program like Photoshop. - Fabe
I have been creating my work outside of iBooks Author, using a non iOS typeface and exporting as a png image file. They've worked fine and each png is about 20-40KB in size so its all good.
I also imagine this method may make it easier to upscale for when/if the next ipad comes out with double the resolution. Just a thought...
If you use image-as-text you should try to make some efforts to ensure that users with disabilities have some form of access to your content and test the experience yourself.
Be sure to set an Accessibility Description for each image with the missing text for user with screen readers. Also be sure to provide the same for any trigger you're using to provide the actor's voiceover if it can be interacted with.
You can test with VoiceOver on your iPad and should really try your book with VoiceOver Screen Curtain enabled.