The way the spell is phrased, devices are mentioned to be "resisted" in the same way spells cast from outside are described.
So that sounds like fizzle, if device Penetration is insufficient.
Looking at how devices work otherwise, what the differences are compared to spells: A device always works, regardless of Wound- or aura penalties. However Aura always modifies Penetration (as well as MR). So a device used inside a cathedral, has 3x aura as penalties, and the Archbishop with has a modest MR from his Commanding Aura, with Aura as a bonus. So the device may prove useless, unless designed with sufficient Penetration. But against anyone without MR, it works just fine.
Do we want devices inside a foreign Aegis to work this way, or do we want the Aegis to actually be good at defending against this?
And does it nerf devices?
The more I think about it, it makes sense that devices need to Penetrate versus the Aegis to work. That way an evil magus can't just churn out simple devices of mass destruction to mundane goons. They need to be powerful! So the magus may be better of going in himself and casting spells.
And IMHO this does not make devices less valuable, they still have merits.
But using a device inside a foreign Aegis, with a Penetration sufficient to actually work, then I assume the MR of any targets is treated seperately, meaning the MR and Aegis levels aren't added.
Also, is the Penetration of the Aegis only relevant for preventnig creatures from entering?
I'd much prefer it if it was the Penetration, and not the level, of the Aegis which the Penetration of the spells from outside/devices needs to exceed. As I mentioned, this is what Muto Vim does, for affecting the spells of another magus who is not cooperating.