I'm also of the opinion that an intellego focused magus is just about the scariest character in the game to another magus. Look at how easy it is for a magus to avoid conflict; Changing your appearance and sound is spontaneous for almost every magus even just out of apprenticeship, invisibility is common as are short range teleportation spells. If a magus wants to hide they pretty much can, except against an intellego focused magus. Look at peering into the mortal mind, that's only level 30. There is a target hearing version of posing the silent question in Magi of Hermes think of how easy that makes finding information. Look at the quaestiorial spells in true lineages, intellego can detect spells that you cast 9 months ago and figure out what they were. Look at the assassin chapter from hermetic projects intellego can find a stray hair or a drop of sweat, it can find arcane connections of you.
An intellego magus can find where you've been, find arcane connections to you and find out everything they need to do an arcane tunnel to you and have their way with you, whether that means killing you, rego mentem control, or just peering into the mortal mind.
I've done up both an ignem focused flambeau magus and an intellego focused Gurnicus magus to 120 years out of apprenticeship and, while the ignem fellow can incinerate you instantly even if you have parma 9 and ward against heat and flames cast upon you, I find the intellego focused character much more intimidating.
here are the characters:
https://forum.atlas-games.com/t/adelbert-a-magi-of-hermes-look-at-intellego-and-astrology/8146/1
https://forum.atlas-games.com/t/ranulf-from-moh-now-150-years-post-apprentice/5637/1