The equivalent of "Windows portable apps" on Linux isn't flatpaks (these add a bunch of extra stuff and need some sort of support from the OS) but AppImages[0]. AppImages are still not 100% the same (and can never be as Windows applications can rely on A LOT more stuff to be there than Linux desktop apps) but functionally/UX-wise they're the closest: you download some program, chmod +x it and run it like any other binary you'd have on your PC.
Personally i vastly prefer AppImages to flatpaks (in fact i do not use flatpaks at all, i'd rather build the program from source - or not use it if the build process is too convoluted - instead).
Looking at their architecture they seem to be a pain to run safely (sandboxed). For example, you cannot take away access to mount syscall due to them mounting themselves using FUSE.
Also are they easy to debug? Do they ship with debugging symbols? Googling around shows that it might be tricky.
Personally i vastly prefer AppImages to flatpaks (in fact i do not use flatpaks at all, i'd rather build the program from source - or not use it if the build process is too convoluted - instead).
[0] https://appimage.org/