I'm not an expert on what's out there now. I have used a few in the past, mostly for access while travelling, so nothing that had been turned on for long periods, or, in an office environment/day to day basis. In fact all of the models I own have been discontinued for a few years now.
When you are dealing with portable equipment, you risk damage. So, you have to consider the environment that these are to be used in. Most new models are not inexpensive, so you have to consider replacement costs (you know that will be dropped), and, the eventual replacement of the batteries (hard to get?). A lot of models of WiFi SIP phones have come and gone over that last several years, so you want something that will still be around when it comes to parts and service. If they are to be used on a desk, then you probably want a set that has a desk top charger(docking station) available.
Some people have unrealistic expectations about WiFi SIP and expect them to behave like a mobile phone. If the environment that these will be used in involves a number of Access Points, be aware that a conversation started on one will drop if you move beyond it's range towards a second AP. There is no hand-off, like a mobile, and it may take a few seconds to establish a connection to the second AP, they don't "talk" to two at once.
Some sets are much better at handling things like transfers and a second call and other 3CX specific features. You need to figure out exactly what the customer expects the set to do, what they are planning on using it for, and then make a post, with those specifics. That would narrow down your search a bit.