GL iNet gives better support for newer and more popular products. Products that are popular, like my AR300M has received much better support and new firmware releases then my very unpopular microuter-N300. Multiple times on the N300, GL iNet has pulled released versions of the firmware, as they did not fully test it on this device before they released the firmware. As much of the support is provided by this forum, by regular users, products that have more users, get better support.
Due to chip availability and popularity, some products like the AR300M16 has had a very long life where others like Brume-W (GL-MV1000W) had a relatively short life. As an end user it is very hard to know which routers will have a long life, and therefor a long support life.
Although GL iNet claims to support products for at least 2 years after they announce End of Life, this support may not occur. My N300 which is still under support, it has not received a 4.x firmware release, and the current released 3.216 firmware has multiple open CVEs that may or may not be fixed.
On OpenWrt support, it is important to pick you router carefully. Unless a router already has OpenWrt firmware available on the OpenWrt site, I will not buy it. This is the only way to know for sure if OpenWrt is an option for your router. Otherwise you are just betting that someone will finish the port for it.
Some links to look at:
What is the expected support period for routers
Inconsistent Product Firmware
Broken commitments! USB150
N300 firmware pulled as the released firmware did not support its 1 Ethernet port
Product End of Life (EOL) Notice