No, these two architectures are not on the equal footing. There are x86-specific bugs. But also there are enough happy users on both sides.
Regarding the update, here is one of the x86-specific issues: attended sysupgrade, with squashfs images, does not allow choosing any other version than currently installed. So, if you need to upgrade, you really have to use the Inspector to "hack" the field value that specifies the wanted version.
In terms of performance, both ARMv8 and x86_64 (at least the kind of devices you pointed at) won't have any problems with 300 MBit/s and sqm/cake. Number of ethernet ports is probably easier to fulfill with x86_64 (there are many 4-port x86_64 router boxes).
I've been using OpenWrt on x86_64 for the last 15 months now (switched from an ivy-bridge mini-PC with two ports to a more purpose built 4-port baytrail-d box last summer), with regular sysupgrades (roughly every 2-6 weeks, following master snapshots myself) - and I'm very happy with my decision.
With 300 MBit/s and sqm/cake, you might still get along with mt7622bv (e.g. Belkin rt3200), filogic 830 and probably ipq8072a (e.g. Dynalink DL-WRX36)/ ipq8074a, which would give you internal wireless as well.
I bought one of the barebones Celeron N5105s with 4x 2.5 Gbe NICs for US$140 last November, added $40 in SSD and 8G RAM to complete the package. It's used for experiments only thus far (a PC-Engines APU2 Jaguar AMD x86 box with 22.03 is my main router). The N5105 has been quite solid running the SNAPSHOT builds, and since we only have a 300/15 ISP connection, I've relegated it to "dev-only" status, but if I ever get a faster connection, it's ready to go.