Quick primer on Why The Clans Are Scary:
1.) In 3050, and even for decades afterward, the Clans have perfected several 'Mech technologies that the Sphere was just starting to recover: ton for ton, a 'Mech intelligently built with Clan tech will out muscle almost anything the Sphere has to offer at the beginning of 3050. And even more unfortunately for the Sphere, the Clans like to put their best designs out in the front lines so they can finish off their opponents quickly.
2.) They came from freaking nowhere: Kerensky and his warriors disappeared centuries ago, and faded into myth to return at the Sphere's darkest hour, like so many mythical leaders before. And now, SURPRISE! They're here to kick your rear instead of save you.
3.) Their culture got weird compared to the Sphere due to their warrior heritage.
4.) Their eugenics program worked, and so those superior 'Mechs are piloted by warriors bred for superior reflexes.
5.) And that's not even getting into their Elementals, power armored infantry that can shrug off anti-personnel fire and strike back. Depending on how their commander intends them to be used, a group of five Elementals can mow down dozens of conventional infantry with minimal casualties or alternatively pounce on enemy 'Mechs and execute them by tearing apart critical components... including the pilot.