I canāt think of a single app that requires the Apple Pencil, or any stylus. That includes the best drawing apps. Yes, you will miss out on some advanced features, but the app will work.
The Logitech Crayon does not support pressure sensitivity, but it does support tilt. Whether that will matter to your wife is purely personal. The lines drawn will be just as accurate as the Apple Pencil. Itās terrific for note taking and quick sketches, which is what I used mine for on the iPad Mini.
Iāve played with a few coloring apps, and some support the extra abilities of the Apple Pencil, and some dontā. For the ones that do the extra features are good or bad, depending on how you like to work. If you want consistent shading the pressure sensitive can work against you. If you like a more realistic appearance (something you might get with physical tools) the pressure sensitivity can help. It is not, however, absolutely necessary. There are tool adjustments, that once learned, can help create the more realistic shadings. Just takes a bit of practice and experience.
(note: many coloring apps will let you customize the tilt/pressure features to some extent)
In short, if youāre wife is just casually coloring, I donāt think she is going to care. If she is trying for specific looks, it might matter.
As for the few other styluses that are Apple Pencil compatible, I have not tried them. Most seem to have the same limitations of the Logitech Crayon. They also tend to cost about the same as the Crayon. I did a bit of research, and decided against them, mostly because they all use micro USB to charge instead of Lighting. I donāt want to carry another charger around.
(note: I suspect the reason these styluses donāt have lightning ports is because they are not Apple MFI certified. Another reason to view them with suspicion. Also why they can be a lot cheaper. No quality standards beyond basic safety are being enforced.)
The port is also why when my Logitech Crayon broke, I spent the extra to get the 1st Gen Apple Pencil for my iPad Mini. The Crayon needs to be plugged into the charger (unless you have an adaptor). The Pencil can be plugged into and charge directly from the iPad. That takes away the worry of keeping the stylus charged. It only take a few minutes to charge the Pencil for a couple hours use.
The cheap rubber tipped styluses are just slightly more accurate fingertips. They have all the cost advantages that NSquirrel mentions, but in the end are just a limited as your finger. They do help if youāre tired of rubbing your finger numb; which is certainly an issue with coloring apps.
The in-between active styluses that predated the Apple Pencil are, in my opinion, the worst of two worlds. They are more expensive than the cheap ones, and just enough more accurate to be extremely frustrating when trying to draw. Also their full feature set is limited to the apps that support those features.