Heather Meeker is one of top lawyers in Open Source...most of the other people in this conversation don't have a background in this and should be ignored
Interestingly, most of the posters correctly identify the core concern. If you've invested significant resources into developing React projects, have IP, and Facebook infringes your IP, you're in very hot water.
Most businesses aren't competing with Facebook or patenting business critical innovations in spheres Facebook will be operating in. Using React in all of those cases is a non-issue.
For these people, the license's restrictions seem very minimal, so dumping React in the Open Source bucket seems reasonable.
For others, this restriction is an existential threat. This is why other people have an issue with the license being considered open source.
As mentioned, Heather has represented a ridiculously long list of clients, including many strong competitors to Facebook.
If she says it's her personal opinion, you can take that to the bank.
In fact, given how many client she's represented, i'd say she's probably less biased than most folks. She's pretty much seen and dealt with it all.
I've seen this bias arise in senior practitioners with successful practices across industry groups.
In any event, her person is somewhat tangential to the important point: the react license is a substantial issue to a subset of people and organizations.
It seems like you'd have no patents grant then anyway and you'd already in the position you're in now if you sue Facebook for patent infringement (and are using React).
The first is with the OSS appellation, which should rely upon the permissiveness of the corpus of rights surrounding the work. React claims that it is OSS, but functionally the corpus is very restrictive.
The second is that the grant itself removes any question as to whether there's an implied patent license in a typical OSS license. This lowers FB's barriers to litigating against people using react using patents as well as copyright. The multiplication of rights afforded to IP holders is an issue best discussed elsewhere, though.
The third issue is that even if FB would never weaponize their React patent portfolio, prospective coders may avoid using the framework even if it is the best suited to task for fear of FB extending into their industry. This is not efficient for either party.
Beside, this is not a matter of law but matter of philosophy.