Today's playthrough showed that Metal Slug 1 is not so good as I thought. Of course, the animations, level design, humor and dynamics are excellent. However, the game is poorly balanced.
It's impossible to complete the game without continuing after losing last life, it's unreal even on the easiest level. But it becomes uninteresting to play with endless attempts. You can just push forward shooting non-stop and you will easily pass to the end.
Also, the game doesn't have some interesting plot as in the next parts. It's just missions against the soldiers and vehicles with a couple of so-so bosses. But it's ok like the first episode of a long series.