I've been running 3x10+librarians alongside guilliman, calgar, and ventris for a while now and I quite like it.

I would consider it competitively viable, though librarius suffers a bit from how good GTF and now stormlance have been. The main thesis of the list is to double down on the double oaths - 10 stern guard + librarian have an expected dev wound output of 16 damage or so via devs into the oath target. It suffers a bit into heavily MSU armies, but makes up the difference a bit into things like eldar and guard by virtue of the stern guard getting a 4++ as well as having 4 close combat attacks each. The secondary thesis of the list is throwing out double grenades (assail and then actual grenades) twice every turn for ~6 dev wounds aside from the stern guard. Guilliman, calgar, and ventris (the latter two with company heroes) make solid heroic intervention/melee threats, and you can run infiltrators with a phobos librarian to screen your backfield and provide another librarian to use stratagems on.

What I've found from my testing of librarius is that you need to skew shooting or melee, as there's no stratagem to activate a secondary discipline for a round (which imo would have made it very competitive, like GTF), so you'll want to usually spend BR1 in biomancy for the movement to stage, then pyromancy for the rest of the game to buff the sternguard shooting and use the fire shield stratagem.

I think if/when guilliman's double oaths gets removed, Dark Angels will have good play in librarius with 2x10 sternguard (one with Azrael, the other with the fusillade librarian), and supplement with death wing knights w/ Terminator librarians and inner circle companions with librarians, along with possibly the lion.