Unraid is not confidence inspiring either. It's just more commercial closed source software, developed behind closed doors and with a slow update cadence (~3 months). They have made questionable security choices anywhere you can see, and I have strong doubts that their code quality is any better.
The PHP scripts certainly are a horrible mess, in all ways. For example, shell injection prevention is based on using escapeshellarg at each call site... that pattern is _exactly_ the structural root cause for vulnerabilities like the one D-Link had.
In no particular order, and obviously not exhaustive: Everything runs directly as root - nginx, php-fpm, smb, ... No AppArmor/SELinux. There is no Secure Boot support (especially unfortunate since boot is from USB stick). No HTTPS access to web frontend by default. SMB protocol defaults are insecure. SMB shares default to public. SSH allows password-based root login. Pools are unencrypted at rest by default. They have a checkbox to enable telnet for management! Very permissive iptables rules. Almost any features that real competitors like Synology would officially provide come from third parties via a moderately shady app store.
Note it's not about any of these individual points. I see above as signal that they are not security experts and see security as an afterthought, rather than as something that deserves a team of experts that specifically cares about it.
(There's certainly other fields they also aren't experts in, like UX - their predominant UI pattern is "list of dropdown fields". Even in storage, one could have a longer discussion how their Array feature - the true core of their product -, compares to modern solutions. There's a reason they've evolved cache pools to just pools as a separate thing, and some users do pool-only Unraid...)
That's all quite understandable since it's a small team with only 2-3 coders (https://unraid.net/about). But nevertheless.