tl;dr “Hotlinking is allowed from Wikimedia servers, but not generally recommended.”

I’ve hotlinked to images on Commons a few times, there is no shame to admit it since Wikipedia doesn’t prohibit it, although not recommended, either. But that’s because of potential technical issues, files might be changed, renamed, or removed, then resulting in missing file.

Disregarding the license, Commons has a specially written entry about hotlinking:

It is possible to use files directly on Commons within another website, by setting up a MediaWiki wiki with InstantCommons, so that Commons files can be used as easily as they can on Wikipedia. Directly using a Commons file via embedding its URL (“hotlinking”) is also possible, but is not recommended. […]

You will be redirected to another page for the technical part:

Hotlinking is allowed from Wikimedia servers, but not generally recommended: this is because anyone could change, vandalise, rename or delete a hotlinked image. On your own server, you will have control over what is served.

So, it doesn’t not prohibit you hotlinking, only not recommended, as it states. Furthermore, it clearly used the word “allowed.” But you will just have to take everything might go wrong after you use the URLs in your content, someone might intentionally replace an image with something that you might not want to have on your website once you hotlinking.

There is also another hotlinking option called InstantCommons instead of using direct URLs, which I I am not familiar with this feature. But you will need to use it within MediaWiki, maybe there is an API, but I honestly don’t care. Using directly URLs are good and easy enough for me.

When hotlinking, remember thumbnails might not be reliable, which are automatically generated when needed by the pages on their servers, so use full resolution to safely embed the image:

How to hotlink: on each image’s file description page, there is a link to the full resolution version. Click this link (it will start with upload.wikimedia.org), then copy the URL and use it on your web page.

Frankly, I am not entirely sure about “from Wikimedia servers,” which sounds like only allowing hotlinking within the servers. However, the page is titled as “Commons:Reusing content outside Wikimedia/technical,” kind of confusing. Moreover, it uses “on your own server” for many times, so with that, I believe, it does mean hotlinking from other than Wikimedia or Wikipedia servers.

If you could host on your own, you don’t need to interpret as long as you don’t violate any copyright, then you are good. But sometimes I don’t think that’s good idea to duplicate 1001th time of same file on Internet, even though that would take loads off the hotlinked servers. If you are allowed to do so, I see no harm if you use for honest contents.