I tried out Porygon-Z once in-game and with the right moveset and a boost from Adaptability can make its Tri Attack hit pretty darn hard. I guess the reason Ursaring is NU is because it has to rely too much on Toxic/Burn Orbs with the Guts ability for that mighty Attack boost. And it also has Quick Feet to bypass its slow Speed.
When Ursaring's Guts gets activated, you better watch out.
(It's strange but whilst making this post I've realised how few Normal types there are in OU! Maybe they aren't as strong as I thought...)
I have to say Slaking is one heck of a powerhouse, given how hard it was for me to defeat Norman in Ruby, but Truant is basically there to stop Slaking getting too overpowered. However, it's great for abusing Hyper Beam/Giga Impact, the latter of which helps Slaking's high Attack. And it's not as slow as you thought, and has solid defensive stats too.
Porygon2 is also pretty good when it has the Eviolite, making it outshine Porygon-Z in some fields. But yeah I do sorta feel about the lack of Normal-types in OU, maybe most have to rely on certain abilities/items.