My guess? Nick sees Mouse as equal to them both (him and Anduriel) in one person. Or more precisely Anduriel sees true nature and power of Mouse and lets Nick know (same way Harry communicated with Lasciel's Shadow, probably). Thus definitely someone to be afraid of.
And what would that power be?
Uriel can act only in response to direct attack from supernatural or in attempt to balance the scales, if necessary on the cosmic level, if necessary.
Fallen can tempt and can lend some of their power to their hosts. But that sliver of power is so enormous it twists the minds of average human, leaving them in semi-control of the Fallen, who then can exert their will on the world they hate, but still in very limited way and always through willing human only.
But only Nicodemus can do whatever he wants. He is so frightening because he accumulated enormous experience and skillset without, seemingly, relying on Anduriel's powers. Including health plan, as because he's effectively immortal (via Judas' noose). Did you notice he does not transform into monster at all? Apparently he's doing excellent job being one all on his own... Tessa (IIRC) described Nick/Anduriel team as true partnership. Partnership implies equality... Did you notice Nick does not transform into monster at all?
Enter Mouse. Being a scion he has supernatural powers. Are those powers enough to challenge Anduriel? Is he Fallen's equal? Foo dogs depend on the proximity to source of it's power, and when we learn
which insufferable wisecrack wizard it is for Mouse,
possibly yes, he is. If he can
just some time later challenge one of the most powerful of the Sidhe after Queens (all six of them), Erlking and maybe Odin
and we know Winter's power
is so enormous she is able to challenge Outsiders showing en-masse,
I would say that Uriel's
acknowledgement of Mouse's status as his... fellow professional, even if not in same weight category, as mentioned in one of the answers,
to be sobering estimate.
But also being mortal, Mouse has much less rules to abide by, too.
...Or even less than that. He himself said so: I cheat.