The liability of Unregistered shares? Being DWAC’d or ACAT into account. They need the paper, the worksheet and an opinion letter to make sure the stock they clear or is being sold is legit.
There's a gatekeeper liability component on the |Broker Dealer. Most brokerages won't accept OTC cert deposits even with the paper, opinion letter etc. Regulators are even more strict these days I help clients with large amounts($100,000) deposit OTC certs or help to arrange private Sales. OTC trading isn't what it used to be.
What benefit does a broker gain from doing it and what does it cost them in terms of time, effort, and liability. If you've got a big enough chunk of OTC certificates and are chipping in a significant amount of revenue to the broker then most could probably find it in their heart to make that equation work out for them. If you're a typical OTC punter trying to ACAT $5,000 worth of OTC certificates and you contribute a couple hundred dollars a year of revenue to the broker....the real question is why in the world do you think they would even consider doing an ACAT?
Stock promoters back in the day would send in quite a substantial amount of certs generating nice commissions. Not to mention the raping the B/D trading desk could get away with not having to reflect your offer (promoters didn't care though). Many B/D's would accept these with no opinion letter, available to trade as soon as it was deposited etc. then regulators put the burden on the clearing firm and the noose got tighter in requirements. Opinion letter, Worksheet, Stock not clear for several weeks after deposit. etc.
Fees to deposit Certs can range from 10% to 30% depending on security in some places. . . If someone offers to pay you in Certs, you usually mark up the cost by 50-100% anyways so the fees don't matter as much.