2 things: 1. Base level Mek are 51, unlocked at level 91, with blueprints for higher levels obtained through the normal way (titans or OSDs). 2. They starve without fuel, and need element to sustain themselves.
Do not fight a level 290 corrupt wyvern with a level 51 primitive Mek. Let's just say they self destruct. 40 element plus other mats just exploded. Thank God for glider suite, that shit blasts you straight into the air when it explodes. Still died wyvern got me on foot. Fml.
At high levels they can output extremely high damage, especially with the sidearm it comes with doing extra damage to Corrupted. You can even fit these things in the caves, however it feels as though they have no damage resistance and can take a lot of damage VERY quickly.