We use them at work. And we are a 24/7 operation. So they never get a chance to rest.
No issues that I am aware of
It really depends on the particular CPU they have in them. The ones with x86 processors usually run low-powered variants, but may still get hot under load. Some do have fans. It just depends.
The REALLY small ones (like Rapsberry Pis / Beaglebones) don’t run x86 and typically have Arm processors and are often fanless (and much cooler, though slower).
An upgraded version of that would be something like the Snapdragon chip inside a mini chassis, like https://www.lenovo.com/us/en/p/desktops/ideacentre/ideacentre-mini-series/lenovo-ideacentre-mini-x-gen-10-snapdragon/len102d0040 (basically a Mac Mini clone). That one still does have a fan though. Qualcomm has this fanless reference design: Qualcomm showcases frisbee-like ultra-thin Snapdragon X2 Elite mini PCs | Club386
And if you get Apple Silicon (also Arm chips), they really are that cool and quiet. My Macbook Pro has a fan but it never comes on unless I’m gaming at a high resolution. For regular usage and programming, I have never heard it come on. The Airs don’t even have a fan and they’re fine. The Neos just have an iPhone chip inside them and also don’t have a fan. The Mac Mini, however, does (presumably to allow higher performance when needed), though you’ll also never hear it come on in regular home usage.
It’s really a product not so much of the form factor in and of itself but of the cooling needs of a specific CPU / GPU and the use case it was meant for. The Arm chips in general are MUCH more power-efficient (and thus cooler, quieter) than the x86 chips. The x86 chips probably still have the absolute performance lead, but that’s irrelevant for the majority of home and small-biz use these days (since they’re all so overpowered).
The truly fanless mini PCs are basically smartphones inside a box. Your average smartphone is already way more powerful than computers of yore and way more powerful than most home users need. Apple finally decided to put one inside their computers. The PC world was just stuck without good options until recently because of the Intel/AMD duopoly. But now Snapdragon is a thing, with Qualcomm copying Apple’s strategy for the PC world, so that’s quickly changing too.
Some of these newer fanless mini PCs are using a new proprietary solid-state cooling technology instead of fans: https://www.pcworld.com/article/2203172/frore-system-airjet-how-the-pc-fan-of-the-future-works.html
Moves air with magical ultrasound vibes, something something. How it works: https://www.youtube.com/watch?v=ba2mbeAN8es