When comparing PICO-8 vs Flax Engine, the Slant community recommends PICO-8 for most people. In the question“What are the best game engines for point & click adventure games?” PICO-8 is ranked 38th while Flax Engine is ranked 54th. The most important reason people chose PICO-8 is:
Because it's an all in one product, you don't have to worry about things like how to load sprites.
Specs
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Pros

Pro Takes away the pain of having to wire things together
Because it's an all in one product, you don't have to worry about things like how to load sprites.

Pro Very simple programing language
It's lua with a few modifications.

Pro Encourages building simple games
As a beginner it's all to tempting to try to make the games you like to play, like Zelda or Call of Duty when we should probably be making pong.
Pro Restrictions of tokens count "forces" to write code effectively
When you reach 8192 tokens, you're screwed. However, this limit can teach you to take a look at entire code and think how you could optimize it.
Pro Possible writing code in external editor
Since launch of 0.1.12c version of software, you can now write and include external script using the same directive just like in C / C++.

Pro Perhaps one of the easiest sound editors around

Pro Share the game in a .png file with other Pico-8 users

Pro You can read the code of any game you play

Pro Lots of examples
Almost every game made for the Pico-8 is open source (if you can download the .png, you can see the source).
Also there's the fancy Pico-8 "fanzine" that has lots of code examples as well as other great tutorials in a beautiful physical or PDF form.

Pro Easy to get things going very quickly
Pro Free
Can be used for free until you hit a rather high revenue cap, at which point you have to pay royalties.
Pro Powerful C# scripting
C# scripting is really well made, comparable to Unity, but without any of the legacy cruft.
Pro Multiplatform
Supports most platforms, including Windows, Linux, Android, Nintendo Switch, ...
Pro Modern rendering backend
Has a very modern, performant and beautiful rendering backend.
Pro C++ support
The entire core is written in C++ and it supports writing game code in C++. The interoperability between C# and C++ is also nothing to scoff at.
Pro Supports multiple IDEs
Works with any text editor and comes with proper support for Visual Studio, VSCode and Jetbrains Rider.
Pro Excellent documentation
The documentation is well written, up to date and accepts improvements from the community.
Pro Great and extensible editor
The editor is both powerful and simple. My favourite part is how easy it is to extend the editor with custom magic.
Pro Source code available
The entire source of the engine is available on Github, though not under an open source license.
Cons

Con No collision library or other common things
Pico-8 is very minimalistic and as such doesn't come with really any abstractions whatsoever which means often you end up re-inventing the wheel.

Con Code editor leaves some things to be desired
Such as code completion and automatic indentation.

Con Costs 15$
Unlike many of the other frameworks it's not free. However this is kind of a feature because it means it's much more likely to be supported.

Con Memory restrictions could be confusing or frustrating to a beginner
Con New software
As is standard with new software, not all of the bugs have been ironed out yet.
Con Few plugins
Unlike more mature engines, there isn't a vibrant ecosystem of plugins yet.
Con Not open source
While the source code is available, it's not quite open source
Con Not latest C# version
It uses Mono and is still on some variant of C# 7.
Con No OpenGL support
It only supports modern APIs, namely Vulkan and DirectX. This means that it won't run on very old machines.
