When comparing ASUS RT-AC56U vs Linksys WRT54GL, the Slant community recommends Linksys WRT54GL for most people. In the question“What are the best home routers that support OpenWRT or DD-WRT?” Linksys WRT54GL is ranked 4th while ASUS RT-AC56U is ranked 6th.
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Pros
Pro Great throughput
RT-AC56U achieves a wired bidirectional throughput of up to 1250 Mbps. For wireless throughput the 2.4GHz band provides an average up/down throughput of 110Mbps and the 5GHz band provides an average up/down throughput of 525Mbps.
Pro Supports multiple DDNS services
Pro Has a built-in VPN server
Pro Doesn't allow sticking with default passwords
Pro Affordable
Cons
Con Potential instabilities with its 2.4 GHz band
Many reports indicate stability issues with the 2.4 GHz band. While ASUS acknowledged the issues, the firmware fix they issued that was supposed to fix these issue has had limited success.
Con The UI can sometimes be hard to navigate
Con "Download Master" feature doesn't work well with BitTorrent traffic
Con Quality of service functionality in automatic mode doesn't do much
Con Not compatible with 802.11n or 802.11ac
The router being a lot older only operates on the 2.4 GHz "G" standard, and won't offer the newer 5GHz standard
Con No longer supported by OpenWRT
This device is not able to run current versions of OpenWRT. For details, see: https://openwrt.org/toh/linksys/wrt54g