One of the many fantastic features of online digital facsimiles is that you can inspect primary sources at length without being tied to the opening hours of a research library that is inconveniently located on another continent. It's important not to let ease of access lead to overstating the significance of online editions, though. What's online right now is not the same as what was printed five hundred years ago.
I've been keeping track of practica facsimiles for some time. I've undoubtedly missed some, while a few I've recorded are not actually available online. Still, the following graph gives a pretty decent overview of the progress that digitalization projects have made, at least for one kind of early modern pamphlet. The red columns show the counts of practica editions per decade (primarily from ISTC and VD16/17), while the blue columns show how many of these have been digitized. The years on the x-axis mark the end of a decade.
As you can see, there's robust growth in the number of editions from 1521 to 1550 that is not reflected by the digital editions. In fact, coverage is pretty meager from 1501 to 1590, but excellent for the decade of 1591-1600. The situation after that is truly mixed: very few are available as complete facsimiles, but VD17 provides Schlüsselseiten for nearly all of them, so you can at least see the title page and incipits.
The average percentage of editions digitized for all 150 years is 17.5%, but coverage is highly variable by decade:
The German incunable digitalization projects have done pretty well. After that, coverage is much lower except for the 1590s. The reason, I suspect, is that the historical transmission of practicas frequently took the form of compilations of dozens or scores of pamphlets that were bound together as a single large volume, and often the pamphlets are mostly drawn from a few years or decades. Half or more of the facsimiles in each decade are from the collection of the Bayerische Staatsbibliothek, with the exception of the 1470s and 1550s, where none of the facsimiles are from the BSB. I suspect that the excellent coverage from the 1590s, with some 80% of the facsimiles from München, is the result of the BSB digitizing one or two compilation volumes at some point. The Stadtbibliothek Trier has recently released several more practica facsimiles from the 1590s via dilibri.de, so the imbalance is only growing at the moment.