This came up every few months when I was at Twilio (2012-2014) - primarily regarding supporting faxes. From what I remember from the folks who knew SIP really well, the issue was mainly that there's too much variability in terms of latency and the modulation/demodulation of the analog signals that modems use.
100% of the companies that I installed LG IPECS systems into back in the day also became e-fax customers.
By the time you've got a dedicated channel for the fax machine (missing an inbound PO delivered via fax because the sales team is hitting the phones hard doesn't fly anywhere) and are paying an additional monthly fee for another DID the cost is usually a wash anyways.
It's money exceptionally well spent to not have to worry about G.729 jitter and the nightmare that is analog>digital>analog>digital and back.
