We have thousands of phones out in deployment, and here is what I know from it, ive been doing SIP/VoIP deployments for ~10 years, including time before we got into 3CX as a platform..
Cisco phones are a last resort unless the actual pbx/host is cisco, their stuff is filled with little proprietary nooks and crannies, just because they preach it as being industry standard, and "better" it is not.... And im a long time student of their training courses mind you.... Cisco's mantra is, lock in, lock in, lock in, and then milk for licensing, it will never change. those proprietary bits are part of their lock in, so that certain things dont work, without the cisco backend, or they are made overly difficult.
Polycom, i have lots of experience with the vvx400-600 series, and i hate them, for numerous reasons, but ill take them over cisco in many cases.... We usually run into them pre-existing from a prior vendor or system, and the client is trying to cut cost by reusing all or some of them, which may or may not be vendor locked. Even once they are unlocked, they aren't reliable without an SBC, the usability of features on them sucks compared to others.
Yealink phones work as if they were designed for 3CX, because they utilize open standards, and the team that writes their firmware seems to have a well established code base. We spend the least time setting up, and supporting Yealink compared to all others. The T5 series are especially solid.
Our sales team does a good job and the issue of a customer never having heard of yealink, is never even on the radar of being an issue during a deal, after all just because something is seen on TV, or in a movie does not mean it works well, or is a good product.