They've basically got melee combat covered. They have both pincers and a stinger, and they have chitin armor on the arachnid portions of their bodies. I imagine that they'd be able to move surprisingly quickly, too; they wouldn't need horses or beasts of burden to act as cavalry. Throw on a breastplate, a helmet and some gauntlets, probably take a bow or a spear for ranged attacks (maybe a shield) and you have a "foot" soldier.
For modern warfare, replace the bow/spear with a firearm, and put on a kevlar vest and a helmet. They would need vehicles to keep up with human ones, not to mention better armor and equipment to deal with the nastiness we have such as flamethrowers, tanks, chemical weapons, armor-piercing ammunition, and the like. Their pincers, claws and chitin would still offer an advantage, but it wouldn't be nearly as useful for them anymore; in medieval warfare, they could charge a group of swordsmen and crush/sting them, but if they tried the same tactic on the modern battlefield, they'd be shot to pieces. They still have fleshy bits, and I doubt even the chitin could withstand modern ammunition.
I wonder what a demihuman-designed car would look like. And, on a related note, a demihuman-designed tank.