There were a lot of similar pen-based PDAs and tablet PCs released around the same time as the Newton. Though I think all of them were as commercially unsuccessful as the Newton. E.g. the Amstrad PenPad. The Newton was not really something people compared to, though - it got compared more to PalmPilot and similar PDAs. It makes sense to put it in the mix of precursors, by all means, but at the time people didn't really draw that parallel.
The "second generation" tablets were very much treated as a separate category from Newton, PalmPilot, PenPad etc. on one hand, and Tablet PCs on the other hand, in that they tended to be built around internet connectivity and browsers as more important even that portability (you were expected to use them around home or the office), and with media consumption as an essential element. The marketing assumption was that people would have a PDA for work, and tablets would be media devices that were largely separate things.
The second generation devices, including ours, and Ericssons, and a number of others, flawed in that they were still too low res (though much better than the "first generation" like the PenPad or Newton), too expensive, and too slow to be attractive for anything but techie early adopters, and with abysmal battery lifetime which meant they couldn't compete with the PDAs, and you were basically chained to home. This was also before wifi was widespread, and various alternatives were still competing for the wireless space - e.g. ours had a data extension to DECT.
Apple's greatest stroke of genius with respect to the iPhone and iPad wasn't primarily technical, but to recognise that it was too early and the hardware wasn't up to it, and wait instead of releasing a sub-par product. "Everyone else" launched tablets as the next step after PDAs around '99-'05 or so, didn't get any traction, and gave up or failed and then a lot of us subsequently laughed at Apple, because "everyone knew" something like the iPhone even wouldn't work.
That's not to say I assume we'd have competed if we'd waited, but we'd at least had a much better shot. There was a very distinct failure to understand the gap between the quality that was exciting to a tech audience vs. what would be exciting to a wider market... That was a very useful lesson.
