For me, the lack of RMerlin firmware would top the list. By far.
Next, the lack of at least 2x 2.5GbE ports (or better), and configurable to WAN/LAN as needed is a huge disappointment in a 'flagship' product. This effectively makes this (still) a 1GbE router, overall.
Finally, the same SoC used in the RT-AC86U, RT-AX88U, and the RT-AX86U and with only (still) 1GB RAM is a slap in the face for a 2021 product of this caliber.
Given the above, it is still the highest spec'd router 'done right' so far.
8x Antennae, giving 3 'real' radios/bands of concurrent Wi-Fi on a best-of-class product, there is no doubt it is superior to even the RT-AX86U.
If/when an AXE model shows up that corrects the obvious deficiencies above with RMerlin support, I hope to be one of the first to try/test it too.
Right now, with a 1Gbps symmetrical ISP connection handled by 2x RT-AX86U's with 2x QNAP QSW-1105-5T 5-Port Unmanaged 2.5GbE Switches have my network humming like never before (the 2.5GbE being particularly useful for the 2x QNAP TS-653D's that handle my storage requirements, on-site, and off).
Give me an RT-AXE86M ('M' for
max) with 2GB or more of upgradeable RAM, internal SSD storage capability (SATA or NVMe), ALL ports at 2.5GbE speeds, or better,
and RMerlin support, and the RT-AXE11000 will look like the beta product it is.
(I know, I've been asking for such a router for years, and I'll be waiting for years more).