Discord is a messaging platform, popular among people I know. I do not use it, and have long refused as such, to the bewilderment of my peers. My reasons are as follows, in no particular order.
Discord is proprietary, closed-source software. The program one uses to access it (the client software) has many flaws. I (and others) cannot examine or modify the way its programs work to fix those flaws, nor can we make or use an alternative program to avoid them. It is, in theory, possible to reverse-engineer it, but that takes unreasonable effort and (at least in the case of Discord) incurs a legal risk.
Many other communication systems are based around open standards, such that any appropriately knowledgeable outsider can come along and make their own client, perfected as they like. In those cases, there usually exist several good client programs, some of which are open-source as to be modified by the user.
The only fully effective Discord client is web-based — you may think it's a local app, but that "app" is Electron, i.e. a constrained web browser. It demands vast computational resources: hundreds of megabytes of memory and so much processing power that it noticeably lags on full-size computers from the 2010s. That makes it impractical on old or low-end devices, which I usually use. These inefficiencies are typical of web apps, and of apps which, having young technology enthusiasts for many of their users, don't worry about working on old computers — Discord is both — and not necessary in principle. Yet even if there were an alternative client, the complexity of the platform, what with its continual use of images and its high-information large-scale concurrencies, would make any client for it similarly demanding.
Discord's only explicit ways to make money are by an optional subscription upgrade and a game store. Neither of these are popular. Considering the costs to run such an elaborate platform (they have voice and video chat, and let users host multi-megabyte files), this can't be sustainably profitable. They probably are, or will be, semi-secretly selling users' data, destroying any privacy-from-corporations any users hoped to have, considering how much can be revealed in what many use as an ubiquitous communication medium.
My unusual phone can't run Discord in any form. If I used Discord as one of my main methods of communication, the quickest way to access it would be by laptop, which is much slower to get out than a phone, and not with me as consistently. Granted, I chose the phone, so I "could" have chosen a more conventional smartphone, on which Discord would work — but I like this phone, so it stays and Discord goes (er, never comes).
As a colourful multimedia superstimulus that combines the actions of dozens of concurrent users in any one place, as with any social media, Discord can easily addict one who uses it, pressuring them to keep up with an excess of seemingly-personally-relevant information. I prefer to keep my communications targeted and unexaggerated.
There is at least one other reason I don't use Discord, but it is unsafe for me to reveal it. My action (well, lack-of-action) may look unjustified to you, but Discord is more repulsive to me than you realise.
I use any of these for symmetric remote communication:
The exact choice depends on the preference of those with whom I converse, among other contextual conditions.