What risks are there with mixing SSD models in RAID?
The single biggest thing that crosses my mind isn't SSD-specific: that the biggest danger with RAID is that all the devices in any given RAID are often purchased from the same manufacturer, at the same time, and therefore tend to get to the far end of the bathtub curve and start dying at about the same time. In that sense, buying from different vendors is not only not a bad idea, but best practice.
You don't say whether you're doing hardware or software RAID. If it's hardware, you have the issue of whether the new models are supported by the controller, both from a hardware support contract standpoint and an "it's too new for me to talk to / my programmer told me not to talk to you" standpoint. Either of those would be a reason not to do it.
There is also the issue of capacity: if you're adding devices that are smaller than your existing ones, even if by only a few sectors, this will not go well. Check the absolute raw capacity to ensure it's greater than or equal to the devices you're already using.
But assuming you can get past those issues, I think it's generally a good idea to do what you're planning.