I heard, Mandamus might not work if your application is with CSIS, since Mandamus is for IRCC not CSIS.
Every application is different and we can't say how a specific one for an applicant stuck with CSIS will go, but the citizenship application (or any other application sent to IRCC) coming to an end is IRCC's responsibility. They can't get away with "well, it's in someone else's hand" for an infinite amount of time.
Both IRCC and lawyers / judges are well aware of this :
https://www.canlii.org/en/ca/fct/doc/2005/2005fc803/2005fc803.html
Check point 21 in Hamalipoor vs Canada.
This is not to say that it will work in a given situation for a specific applicant after a specific delay (and in Hamalipoor's case, we're speaking about a frankly ridiculous 5 years wait), but given the jurisprudence, absolutely nothing stops an applicant stuck in BG to consult with a lawyer and start the process. They should have a better feel about how it will go.
Good luck to anyone going that road, or anyone stuck significantly longer than the expected time really!
And if anyone goes for it, please let us know the details, should help future applicant googling this same thing