So basically the title.

Is it up to reaction time or is there a trick for it? I've never once used it since I'm afraid that I will mess it up and proceeds to get one shot by whatever it is I'm fighting.

Also, does it block all attacks or are there some exceptions?

Timing. Hit and hold your bock button around .5-1 second (from experience). Slower attacks such as Gargoyle attacks are easier to guard compared to Satan/ Lucifer's AOE lighting/ holy attack. Also from experience you should be able to guard at least 90% of all attacks.

Attacks that can hit you more than once such as Nue's spin kick and aburasumashi's (however you spell it) flame pilliar attack CAN be guarded for say first 3 hits then your just guard will turn into a normal block.

Side note: Slow connection/ high ping can fuck up your guards.

As long as you have good connection, low ping and no FPS lag, you should be able to Just Guard quite easily. Aburasumashi (you spelled it right Tsumi) are good to train against with their fireballs. Kappa tend to make for good training too, as well as Ittanmomen and Nurikabe (Nuri is especially good because it's slow as fuck, so you've got time to prepare). My Just Guards are a bit hit and miss, but I ten to get heinous FPS lag even on low settings. If I tried to take on Satan or Lucifer I'd probably lag out of the game.

There is another benefit that's rarely mentioned: a momentary burst of SP regeneration occurs when pulling off a Just Guard.

Requires practice, that's all.