Skaven individually suck quite hard
They are also cowards to the core indeed
Nonetheless, the best unit in the book (and maybe in the whole game) is SkavenSlaves, the cheapest unit all around with an effective Leadership of 10 with re-roll if you play it correctly (Ld7 from General bubble, then +3 from ranks), and their numbers make them Steadfast until they've been slaughtered for a few turns
Most Skavens players are quite succesful as we have lots of excellent tools:
Warp Lightning Cannon
Hellpit Abomination
Doomwheel
Slaves
Poisoned Slingers Gutter Runners
Plague Furnace + monks
the first 3 are all rare and you should often try to max out your rare (2 WLC is nearly mandatory, they're unreliable but pack an incredible punch when it goes well)
Slaves form your glorious and heroic battleline! (more like walls of flesh your opponent has to carve his way through). Also, you can shoot agaisnt enemis engaged in
CC against Slaves, which is a huge capacity
It's not an easy army to understand properly (thank the drunk monkey who wrote the army book ..), but it's quite efficient when played correctly, and very potent once mastered (even though you can't really master randomness and luck)
Lots of very "pro competitive" players despise them because you cannot expect the Skavens to shine reliably, sometimes they suck and there's nothing to be done, sometimes you go "wow, I'm sorry dude ... I'm lucky today" and your opponent just cries
And as a finishing touch, it's rare to win combat with Skavens, it's normal, our army is weak in
CC and should stall it for as long as possible usually
I recommand you to register and look up at this forum:
The UnderEmpire