Think the best way of articulating why I find this supposedly old/outdated fighter sick as someone who came to it without ever having played a subsequent SNK fighter is the infamous Geese Howard boss fight. In a vacuum, he's as cheap as its reputation would lead you to believe: his Reppuken has damage output that's approximately one third of your health as well as range high enough for it to severely limit your options against him and he can also casually invalidate the attacks you manage to land on him with a throw that does comparable damage to the Reppuken. Prepare to see that screen of your character falling out the window often.
However, the more times you die to him, the more predictable (no pun intended) his AI starts to become. If you jump forward and attack, he will always go forward and attack. If you try to approach him by using a special, he'll prevent you from doing so with the Reppuken. If you stand still, he'll do the same for a bit before making his move. So you have to pick up on every little quirk on his AI and find out how to bait him into a situation where he gets hit. While the sheer damage of his heavy hitting attacks would seem unfair, getting one hit on him more often than not means throwing him out of his comfort zone and turning that one hit into multiple, thus doing a similar amount of damage to what he would have done to you with one hit. If you keep this up, you can even get him into a situation where your special move will land on the early frames of his Reppuken, interrupting said attack. Through this plan of attack as well as, my noob at fighting games ass was able to eventually 2-0 him and throw him out the window like he did to me 28 times.
With its small player roster of three characters and its arcade mode that faces you off against opponents designed with no expectations of being playable, Fatal Fury 1 feels like it was designed as a single player game first and with multiplayer as an afterthought. And I am being 100% unironic when I say this design philosophy makes me feel like I became a better fighting game player, be it through learning how to exploit the habits of seemingly unbeatable opponents or by building up the ability to press buttons faster to make the most of those small moments of vulnerability when they pop up. As someone whose engagement with fighting games has usually been respecting them from the sidelines and/or having a quick laugh in a run of arcade mode on the lowest difficulty, I truly get them now.