So, I get the argument --- there's hardware, why not use it --- but it's got some things that I'm not super-enthralled with, even though the author has clearly put a lot of work into making it work.

• I don't know how far I'd trust the Linux side of things to automatically come up and stay up. Like, they use a program to trigger Termux at boot and disable battery management to try to avoid having Termux be killed, but the host OS isn't really designed for this.
• Stuff like the network is outside the control of the GNU environment; Android apps can only do so much. They did mention some of the issues here and looked to work around those limitations.
• I don't know if Android apps are allowed to reboot the phone; I would guess not. My Termux environment doesn't have a shutdown or reboot command, which also kinda makes me bet not.
• Android is a lot more amenable to doing things like expecting there to be a user at the screen. Say the phone reboots to do a system upgrade and prompts me for something, which is reasonable to do for a phone but not great for a server. I can't do much about that remotely.
• I don't get to update the kernel, and at some point, phone vendors stop shipping updates.
• I don't have access to all of the hardware.
• Phone hardware does have a battery, which is neat, but it's also missing hardware that I'd like on a server, like an Ethernet socket and much (or on many phones, any) by way of expandable storage. No external display port (which, given the Android expectations about a user being present, might be something that one might want to have a way to access remotely). Might be possible to add on some of this via USB.
• No convenient way to remotely do a hard reboot, which is something that I'd like on a server. That integrated battery backup also means that you can't just yank power. I mean, maybe you could rip the power button off and rig up some sort of relay hooked to the contacts or something to hold the power button down using some other system, but...

I think that there might be reasons to do this. Like, 14-year-old-me might have done it because 14-year-old-me has little disposable income, but plenty of time to work on things. If an old phone fell into my lap then, hey, sure. And it's got interest from an "I'm gonna do this, dammit" standpoint. But in terms of saving money...even with memory prices where they are, low-end, small PCs are not that expensive. Here's a system with 8GB of RAM and 512GB of SSD for $180. Here's a refurb --- and it is a refurb, but the phone is also used and EOL --- for $129 with 8GB of RAM and 256GB of SSD. Those have the ports above, can just run straight GNU/Linux, run x86, are more expandable, are designed with less constraints on power and cooling, and meet or exceed the phone specs. Most of the people who are in a position to hack together a custom server on a phone and are comfortable fixing or working around any issues are also the kind of people who probably are in a position to pull a pretty solid income, and where the time tradeoff becomes increasingly unfavorable.