How safe are the North-Eastern provinces of India, like Tripura or Nagaland? I keep reading articles about some uprisings but they are years old.
Is it better now? Has anyone ever been there?
As a local it'll be different, as you're living part of any conflicts.
However, as a traveller sometimes you're at risk because you're naive and have no idea about how to act, where to go and what not to do in these situations. At the same time you're also sometimes not a target, depending on the circumstances.
In these times, the best place to go is various countries' government travel advisory pages.
For example, the US State department has a list of current countries that they provide travel warnings for. Some of these you'd expect from news; places like Mali, Syria or Yemen, but there are also some surprises sometimes - for a while the Australian travel advisory site listed New Zealand as a place of warning - in post-earthquake Christchurch.
In this case, the US State Department site doesn't list India at all, so they're not concerned about you travelling there.
However, I tend to look at the NZ Safe Travel Site, also a government site, as they list their assessment of EVERY country. In the case of India - their page on India actually lists a few major concerns, and in the case of areas you're asking about:
There is high risk to your security in the north eastern states of Tripura, Manipur, Assam, Nagaland and we advise against all tourist and other non-essential travel to these states due to the threat of terrorist activity, violent crime and demonstrations.
Their page was updated today, so it's fairly current.
I found this thread of interest as I am a tour operator based in Northeast India. I would say, as a foreigner you are very safe in the region. We routinely receive guests who come from all around the world and till now, I can assure you our guests have not had any problems related to terrorism or violent crimes.
The advisory pages of various governments will always take precaution and not be in sync with the real situation. Northeast India is much safer today than what it was 6-10 years back. And as a westerner, I am sure you will have no issues going to even interior places as the people here are very welcoming to tourists.