You won’t be able to port the apps from one to the other, is the main thing - and some Apple-targeted tools don’t have equivalent parallels on Android (and vice versa). Most do, one way or the other: e.g. my husband and I have separate sleep tracking and fuel tracking apps; his iPhone fuel tracker is better than my Android one, but my Android-based medication tracker is better than his; another example is the 1Password implementation which on my Android is read-only, so I have to make entries on a PC or on my iPod Touch.
In addition, the difference between an Apple computer and an Apple handheld / tablet is pretty substantial; you can’t port apps from the iPhone to the computer.
All platforms have tools that can open / edit documents, PDFs and spreadsheets. All of them should be able to watch Netflix movies etc. There should be apps on any of them that can handle ebooks from any of the major purveyors. File-sharing tools such as Dropbox will likewise have parallels on all the platforms.
So - while it’s convenient to have an Apple-based tablet if you have an iPhone, for example, it shouldn’t be a show-stopper. You may have to lay out a few dollars to duplicate the apps on the second platform.
And I concur on the Coby tablet. No experience with those, but we were given a couple of pairs of their earbuds and they were crap (one didn’t work at all, the other failed within a week or two). I don’t think it’s a coincidence that the Coby logo looks an awful lot like Sony - they’re hoping people thing there’s a connection or that they actually misread the label.