The second anniversary title screen looks unoriginal.
No cloud or any backup features, unlike Metal Slug Defense.
The game butchered up the plotline of the Metal Slug universe (that was already messed up in Metal Slug Defense) even further.
The game is still currently unbalanced and easily prone to powercreeps, as many units added currently are able to easily wipe out enemy units with their special attacks.
One of the most noted and most criticized aspects about it are its new characters, which absolutely do not fit in the Metal Slug universe, since they are heavily oriented towards anime crowd and not in a good way, with most of them looking completely absurd.
The game is yet another gachapon game filled with microtransactions, as many units can be only obtained via "Crank Box" (a kind of gachapon featured in the game).
Also, after a few weeks when a new character is released, the developer release a "special" version of the said character which is much stronger or more useful than the original version.
and if you're also counting the Iron (a modified variant of Iron Nokana featured in Metal Slug 4) and its reskinned variants, there are 11 units based on the Iron Nokana in the game. For example, the Iron Nokana, one of the bosses from the first Metal Slug game, has 5 reskinned variants ( 6 if you're counting the original variant).
Talking about reskins, tons of units featured in the game are essentially reskins/palette swaps of existing units with much-improved stats compared to original units.
The game is essentially a reskin of Metal Slug Defense with some tweaks and contents, to make this much worse, SNK released this game as a tie-in to a 25th year Anniversary of the Metal Slug franchise!.