Just to expand a bit on Vaz's great info, the reason Tomb Kings have good archers is that in Fantasy (not sure about 40k) there are To Hit modifiers for ranged attacks. If your enemy is further than half it's range away then they get a negative modifier.
Let's say Skeleton Archers have 18" range. If the enemy lies MORE than 9" away (more than half of the Archers' total range) then normally they would take a penalty vs hitting the enemy. However with TK, their archers NEVER take any sort of penalty against their ranged attacks, which is huge in 8th edition.
The Tomb Scorpions are amazing as they can just crawl underground toward their enemy then BAM! In the face! No risk of being shot at or destroyed before then.
I also really love the Ushabti. They have incredible models (I love Anubis!), incredible stat lines, and they're just fearsome on the battlefield. They cost a lot of points and a lot of $$ though.
However, as Vaz mentioned, TK's signature unit is their Chariots and Cavalry in general took a big hit this edition. After the initial charge, they kinda run out of steam and can die pretty easily after that.
Tomb Kings are not unlike Vampire Counts in that both sides lose models if they lose combat. They never run away from combat, but if they lose combat their models just turn to dust and disappear. That being said, VC And TK are the only armies with natural spells to resurrect their units, which is a huge deal this edition, just as it was in 7th.
As Vaz pointed out, the TK spells (Incantations) are very easy to dispel right now, however you'll never lose a Liche or King to Miscasts, so you can throw as many dice toward casting spells as you like and never worry about backlash, which is a massive boon to you and not so much against your enemy.
One thing to be greatly aware of is that the TK army suffers from something called... well I forget what term they use, but basically Fire is a huge danger to TK and if you're fighting someone even marginally familiar with TK, they will bring as many Fire attacks and spells as they can.
Currently the TK book is pretty old and doesn't really line up with 8th Edition. The TK units cost way too many points for what you get and some of the abilities need buffing. With that being said, there are rumours swirling all over that Orcs & Goblins, Tomb Kings and Ogre Kingdoms are next in line to get their books first for this edition (all rumours of course).
So with all that said, TK can be incredibly fun to play, but right now you are going to feel very underpowered in most respects. If the new book turns out to be amazing, I have no doubt TK will be a top tier army and a beast to be reckoned with!