Another 'in a nutshell' post...
All Ogres are strong and tough so to be included in their army an Ogre commander should ask himself "what makes this unit worth including in my plans ?"
Yhetees can be important because...
They treat difficult ground as open ground, yes that includes woods.
They have 2 additional profile characteristics that set them apart from Ogres. They have an initiative of 4 and a move of 7.
Yhetees are an Ogre Kingdoms's cavalry. They will usually charge into combat and strike before most humans (ie Bretonnians and Empire), Dwarfs, Lizardmen, Undead, Orcs and Skaven (on a roll of 4+)
There is a major drawback, Yhetees are very vulnerable to missile fire. Toughness 4 and no armour, ward or regeneration save. They are arrow magnates !
Because they cause fear and are fast hard hitters I would take a unit into combat but not against any type of Elf or any enemy you're expecting to bring a lot of missile troops.
Note: they are hard to hit in close combat; the enemy suffers -1 to hit modifier due to "ice breath" - although, it's got to be better than Troll breath !
Image is © Games Workshop Ltd. All rights reserved. Used without permission.