Most people have had good luck with the Iris outlets (really a Centralite Zigbee outlet plus a Z-Wave repeater; the outlet itself was whitelabled for other manufacturers including SmartThings). However, some people have had bad luck with these, and apparently the luck corresponds with the firmware version. You can find a few posts on the issue, including this: Issues with Iris Smart Plugs as Z-Wave Repeaters. I think SmartThings can still update the firmware on these (obviously Iris can't anymore), but I'm not sure of anyone who's tried.
Personally, I don't mind buying separate devices for each protocol; that way, I can try my best to optimize the location of such devices for each protocol (without RF goggles, whether I'm doing a good job is a different story). It's also not 100% seamless--that Iris outlet has to be paired as both a Zigbee device (the outlet) and a Z-Wave device (the repeater portion, the driver for which doesn't really matter but it does need to be paired or it won't do anything), and there's a separate process for each, so it's not always intuitive. I also imagine it's easier from an engineering standpoint not to cram two radios and two antennas into a single device, especially smaller ones, and you're likely to get better performance by a device where you don't have to work with those constrains (just a guess).
But yes, I agree that in some cases it might be nice. Personally, I don't have any problem filling my network with enough devices for both protocols (a mix of smart outlets and switches of both kinds work for me). To each their own! If this is what you want, though, I'd probably look on eBay sooner rather than later--with Iris having gone under, I'm guessing they'll become hard to find (or expensive...or maybe extremely cheap, who knows) after a while.
