I don't have that APU and mainboard combination. But I have a 4 core / 4 thread Intel Haswell i5-4570S which runs at 3.6 GHz and a GTX 960. So an entry level system by todays standards. The AMD 3200g is on par with my system from a CPU standpoint, give or take a bit. GPU wise, the GTX 960 is faster, especially after increasing the resolution to 1080p and beyond.
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with this setup and Windows 10, Youtube 4k and general 4k videos run all flawlessly. It is a very snappy system with an SSD. When it comes to gaming, the 3200g is worse than the 3400g and both of those will struggle with 1080p gaming @ 60Hz and good settings. So 1440p system should be run at 720p and the 4k system should be run at 1080p with reduced details (depends on game). Maybe eventually we'll get integer scaling from AMD and Nvidia, that will help with those resolutions on those monitors. For APU use, you should really invest in good memory. 3200 MHz is the lowest you should get, scaling goes even higher (3600 MHz is still very possible on this Zen+ APU). Those APUs make a lot of sense in small systems that are size constrained, don't game a lot or you have a very specific budget. Otherwise, a normal CPU (Ryzen 2600 for example) with an older, used graphics card is a great way to get a budget system for a good gaming experience.
For that video, I think the mATX build has the wrong case link. That is the same mITX case and clearly does not accept a ATX PSU. For mITX APU use, I would take a look at a case that has a decent SFX PSU built in (in the 250 to 350W range) and enough space for at least the AMD stock cooler. I've personally grown to like Jonsbo / Cooltek cases and have their mATX one here myself. If you want to go as small as possible and not overclock the APU, the ASRock DeskMini A300 is great and should support the Zen+ APUs after a BIOS flash (it is a case, motherboard and PSU all in one). And if you are really budget contrained, the 2200g and 2400g dropped in price (2200g vs 3200g delta is 25€ in Germany and 2400g vs 3400g is 40€) and aren't that much worse. I'd personally get a 110€ 2400g over a 98€ 3200g: 4 more threads and better iGPU, a few MHz less and slightly lower IPC.