After reading the comments on this website and other, I didn't expect anything but an empty movie à la Marvel, with no content, half a story but good SFX to make us swallow the pill, but I was actually surprised, in a good way.
That might not be the best MIB movie, but I would place it in third position, as I found it better in many ways than MIB3.

If I had to state the number one reason this movie flopped, I'd probably blame the actors, particularly Tessa Thompson. While her character was inconsistent: borderline annoying at the start of the movie with her way-too-much-excited attitude to not-even-surprised-of-alien-stuff-everywhere fro the half of the movie, far from Will Smith's Agent L discovering the system on screen with us, it feel that Agent M was recruited on screen, acquired experience somehow and came back finishing the movie as a more mature agent... Plus she didn't really fit there, with no chemistry with the rest of the cast, it's more of a movie following two agents (and Pawny) than a duo.

Chris Hemsworth on the other way is a very good half of a duo, unfortunately there's no real second half, and he, or more likely his character can not (and should not) compensate that void alone.

Liam Neeson is probably the best pick for an experienced agent now head of a MIB branch, and play that role to the perfection
[spoiler] even though his over protective behavior toward agent H makes him suspicious early on in the movie [/spoiler].

The movie in itself is not as funny as the 2 first iterations, but has its moments, I can not say jokes fall flat because I did not notice a lot of them and the ones I noticed did at the very least make me smile, so it is not a failed attempt of comedy, but more likely a deliberate choice to bet on well... not humor at least.
The story was not half bad, but was not brand new, [spoiler]the concept of alien(s) coming on earth carrying an object/weapon incredibly powerful but searched but a destructive alien race sounds a lot like the first movies,[/spoiler] ok, there are a few twists, but it does not make it original enough to make this movie memorable, compared to MIB or MIB2.

I think the accent was put on blowing our mind with alien and advanced tech but too much it too much. When there is no reason , no purpose and makes no sense, why include it?
[spoiler]First a teleoperator can track an alien spaceship with a simple laptop while at work despite the incredible MIB tech, they can't fake an asteroid trajectory or erase it from an basic internet website?
Then, the New York subway transforming in a kind of overboosted hyperloop. The idea i actually really cool, but why in the hell did they need to include a physical transformation of the old looking classic metro train into a super advanced one while in both station aliens are roaming around hence the total absence of a need for camouflage. To be noted than in London, the train stay hi-tech...
Next, in the first fight scene against the twins, just outside the club, the car is filled with weapons, litteraly every single part of the car is a weapon cache, even the side mirrors or the exhaust pipe. And if you try to put it all together, you will quickly really, that it i not technically possible, unless the MIB cars would be secretly TARDIS?
Again with the cars, they can obviously transform, to fly, overdrive or any other function, as a classic MIB car should do, but they push the transformation almost to a Transformers level, in the last scene, it takes almost 10 seconds for the car to change into fly mode, while in the original movies, a reactor got out of the boot and that was enough, they here felt the need to make the whole trunk disappear, move, change shape to, at the end, give us a badass planecar! No just kidding, the result looks more like a frail paper plane than anything else.[/spoiler]

I will not most certainly nor beg for a sequel, but I would not spit on it either. I only wish I could see more Agent H and less Agent M in the future.
Idea for a future movie: a bridge between the "real" MIB movies and MIB International where Agent K (Tommy Lee Jones) and Agent T (High T, Liam Neeson) fought together as field agent a few years/decades back, with, why not, young Agents L and H making a cameo...

And of course, last but not least, I loved Pawny, he's cute, fun, badass and put a lot a fresh air in the movie. He's probably the one who made me laugh the most, from his very first appearance, i wanted to see him more, and I still do!

In the end, I enjoyed watching this movie, I had some fun, it was a occasion to dive back into one of my favorite sci-fi franchises. The movie was not really freshly original, nor the actors fitting in, but I can not qualify this movie as a bad movie, it an OK movie, tasteless compared to the rest of the franchise but an interesting entry for the younger generation who might not have been there in 1997 to watch Agent L and Agent K making there debut on the big screen.

I give it a 6.5, and just because of Pawny, I'll round it up.