I remember playing this back then. Here's what I think about it nowadays.
The game didn't age all that well because of...
(Spamming projectile attacks while standing far away from your opponents is for the most part a dominant strategy)
-Frame Rate Issues
(The Wario level for example is prone to frame rate drops, input delays, & broken collusion detection, to a point where I just use the right click then skip function.)
-Limited Move Sets (Only 5 Moves for each character.)
-Some Questionable Character Choices
(Could've choose Eggman, Metal Sonic, Amy, or any Sonic character over those two random fan made ones.)
-Some fan made sprite work could use some improvements by adding proper shading & making sure that the characters are at least on-modeled (I.E. Mario, Luigi, Fox McCloud, Jigglypuff, & Cloud Strife)
The game is flawed but it isn't 100% bad cause there are pros like...
-Being able to unlock more than one characters instead of one at a time.
-2B's voice actress was involved in this game.
-The inclusion of non-video game characters like Goku.
-The inclusion of third-party characters that aren't in any Smash game as proper playable characters such as Crono, Lloyd, Mega Man X, & Zero.
-The option to skip any levels at any time & it's arguably a good speedrun strat.
I honestly prefer Super Smash Flash 2 over the original.