The E8450 suppports WiFi 6, AKA 802.11ax which can support speeds up to 1Gbps. It's CPU architecture is ARM which supports more compiled packages than other chipsets.
UBI Layout
According to the developers, the UBI flash layout is better because it:
Performs wear-leveling to increase the chips longevity
Has a more efficient layout which nearly quadruples the available memory from 20MB (with stock) to 75MB (with UBI)
Allows easier allocation of space for more storage volumes
Comes with the fully-featured U-Boot 2021.04, much more powerful than the default U-Boot 2014.04-rc1
OpenWrt
OpenWrt is an open-source linux-based operating system for routers, modems, and other embedded devices.
More secure because it recieves more reliable software updates than the stock firmware. Because the open-source code is reviewed and improved by volunteers around the world, it recieves security updates long after the manufacturer stops providing them.
Less buggy because there are more eyes on the project, bugs get fixed sooner and new features get added quicker.
More features because you can install any of its many packages. These packages provide additional functionality that may not be available with the stock firmware, such as proxies, tunnels, SIP, traffic control, load-balancing, etc.