My experience is from playing through Hard Mode.

It's very typical for people to sum up a game as 'If you like X, you'll like this' if the game has some sort of failing. This game is a unique case. I would argue that even if you have a good reason to play this (I want to finish the series, I want to play this because I played it growing up, I am collecting Gamecube titles, someone else recommended it) that you disregard that and still not play this game. It is as bad as they tell you it is.

I will provide some advice to those who decide to play. The invisible soldiers can be killed with all weapons, but they are vulnerable to the Flamethrower. I never used Flamethrower ammo because of this, but around the time you stop getting Shotgun ammo you should start using it to supplement your other weapons. The game expects you to return to previous areas to quicksave abuse, you can tell because they purposefully block this method in the last level. The game is 11 real levels and 1 final boss level. You can kill the railgun tank in the first section of its level, you just have to only aim for the turret. Loading a section causes all triggers to fall off, health won't despawn and if you're in the middle of one of the suicide turret gun sections, you can leave the area to limit the spawns. Do not bother sniping heads, there's no point. Switching weapons cancels the reload animation VERY quickly. Don't play this game.