the late 2015 imac is probably the most compelling all-in one computer with a 4k/5k screen, mostly because it's the only (5k) all-in one solution with this kind of resolution. now there is also the asus zen with a 4k screen, which is much more affordable, maybe check this one out.
the cpu is powerful, the ram upgradable and with a 500gb ssd + whatever external (RAID) storage you are using it is a great choice for any visual artist who does not require a hardcore rendering workstation.
you get a fantastic screen (10bit) in a beautiful form factor.
if you go for the 395x gpu you can play lots of games on 1440p without any issues and since there are plenty of AAA games which aren't considered graphics demos with guns, you can play those in 4k/5k if you wish.
you won't be playing the newest and best looking shooter game at 5k and 60fps because since you have done your research you will know already that even the high end dedicated graphics cards the size of a shoe box will struggle getting you the performance you demand or wish for.
is the gpu underwhelming? sure, for heavy gaming and heavy rendering. for 4k video editing it's fine and photography taxes the cpu most, for this it is fantastic. working on 60mp raw photos is a joy, 200mp is fine. if you slap dozens of layers on your images you can expect lagging, which is expected.
if you are a working professional you have probably made the math already but if this computer, compared to a windows machine, saves you only a few hours of discomfort and annoyance which you can use to create something nice and then write the bill matching your work, you will still have spent more money but also made it up already.
if you expect a macbook pro to do the same work than a fully fletched workstation or complain that a build with a "bad" gpu like the imac 5k cannot be used to play the newest and greatest AAA games on 5k, then you should spend more time being actually productive so you can afford a windows machine for your gaming hobby/profession.