The information page for this digital library gives an overview of the collection and related initiatives. I can't find any evidence that there was anything left undigitized from the physical collection, but there is information about non-scientific drawings and portraits that were segregated from this digital collection, and are instead viewable with Harvard's general image search. On the information page there is also the ability to do an advanced search of the collection. Browsing is not as intuitive, with no thumbnails available, and all scans browsable only through the "browse indexes" section of the MCZ Ernst Mayr Library Artwork Collection, which appears to have only the Burkhardt drawings in it. As a new user to the collection, it made me question if I was browsing the right collection, and if I was seeing the results of only the Burkhardt collection. For sustainability, I wonder what would have to be redesigned if non-Burkhardt items were added to the MCZ Digital artwork collection.
There's a large amount of metadata for each object, including biological classification information for the animals represented in Burkhardt's drawings, expedition names, and even the date and locations for most drawings. There is enough information to find the objects in the physical collection, with each item having its own unique call number.
For each image, there are 4 levels of zoom, allowing great detail with an option to save the zoomed image. There's also an option to "View full image". Downloading an XL version of this scan gave me a ≈ half-megabyte .jpeg file, in 72ppi but with very large dimensions (1469 x 2400 pixels at 20 x 30 inches). Since the original image was 9 x 14 cm, I wonder if the scan could also be thought of as ≈ 400ppi at the original size? In both ways of saving, there is an annoyance in that saved images are named "Convertor" by default, rather than referring to their name in the collection.
With respect to the collection's emphasis on searching rather than browsing, lack of thumbnails, and inclusion of biological classification information, I would guess that the intended audience for this collection is researchers who work in the fields of biology and zoology.