The Raijin is an understated robot. Despite having high firepower, physical shielding and 130% firepower in Bastion Mode and the ability to climb, many players seem to dislike the Raijin, most of them complaining that it is simply 'too slow'. This is despite it having many qualities that give it the edge in combat.
However, most of these pilots probably find it difficult to pilot this robot well. Like all robots, the Raijin has its uses, and rewards its pilots greatly when piloted well, and in my opinion, receives much more than its fair share of criticism. The Raijin has an unusually high amount of qualities which makes it a very effective and versatile robot.
Its pros are: High firepower, can climb, firepower boost, physical shielding, doesn't suffer from landing lag, agile
Its weaknesses are: Slow speed and large frame
2019 Update: The Raijin used to be a very powerful robot and a force to be reckoned with in more or less all leagues. However, it is now largely obsolete and I would not recommend using it above Silver league unless equipped with Avengers/Embers. It belonged in an era of slow moving heavies with huge firepower and defense but unfortunately is no longer viable in an era where mobility and ambush tactics (amongst OP abilities and ridiculously fast knife fighting) reign supreme.
Firstly, the Raijin is exceptionally durable, with 250,000 health at level 12 mk1. It has the record of having the highest amount of health for a robot in the game, and has 300,000 health at mk2 level 12. In addition to this, it gains frontal protection from plasma due to its physical shielding system in Bastion Mode. It's incredible armour means that unlike many other robots, it can take damage that would kill most other robots (such as a DB salvo) whilst having enough health left to destroy or severely damage its opponent. Its defensive shielding also gives it extra resistance against plasma weapons such as the Taran, which are widely used.
Secondly, the Raijin is quite agile. Yes, you read that right...one of the slowest robots in the game is one of the most agile. Having 36mph speed at level 12, Raijin definitely isn't fast. Take note though, because at level 12 mk1 it's still faster than the Griffin. However, due to it having four legs, it doesn't suffer from landing lag when falling from a high place. Whilst other robots pause to absorb the shock, the Raijin (and the Fujin) can just keep on moving without delay. Also, the Raijin can instantly change direction without having to rotate its lower body, which all robots except it and the Fujin must do to change direction. This can come in very useful when moving between positions, to avoid threats and to (partially) dodge missiles such as the Aphid and Zenit.
The Raijin can also climb, allowing it to reach inaccessible positions on maps that sometimes even robots such as the Rogatka, Spectre and even the Hover cannot reach, as it can also enter Bastion Mode whilst climbing, allowing it to stay still whilst 'hanging off' a slope. This ability means that it can also go over some obstacles, such as hills, slopes and rocks that are too steep for other robots, instead of having to go around the obstacle.
Finally, the Raijin has awesome firepower, and is an excellent defensive robot. In Bastion Mode, it gains a 30% firepower boost and very durable defensive shielding, in addition to a higher viewpoint. This makes it a ferocious brawler, fighter and beacon defender, performing best with close range weapons such as the Thunder, Ember and Redeemer. However, it also performs well as a sniper, suppressor and mid-ranger, especially when equipped with Zeus, Dragoons, Tempests and Trebuchets. The Thunder Raijin in particular excels as a beacon defender, due to the devastating damage the Thunder does at close-point blank range, which a robot must enter to capture a beacon. Spawning in on a beacon about to be captured in a Thunder Raijin is a popular and effective tactic on Beacon Rush. Although it performs best as a close-range fighter, the Raijin is suprisingly versatile and can be equipped for a variety of roles from brawling to sniping.
Overall, the Raijin is a great robot, performing well in most roles whilst having decent agility and excellent health and firepower. With its Bastion Mode and climbing ability to top it all off, this versatile fighter is certainly one of the best robots in the game.