It should be safe, although I don't know if it's worth upgrading yet because even with 19.07.0-rc2 you still need to apply a speed fix. And there's other speed enhancements being worked on, but we probably won't see them in any builds for a while yet.
Some of the LED options don't behave correctly either. For example, try to use phy0radio so that the LED turns on (not blinking) when the radio is enabled. If you restart the radio from LuCI then the LED will turn on, but if you reboot the hub then it won't.
The default WiFi settings also seem a bit weird. For example, QCA9880 (radio 0) will use "driver default" for the country which limits the power to 20 dBm (100 mW) and despite the width defaulting to 80 MHz it'll use channel 36. However, if you change just the country code to GB then it'll start using a channel like 116 and the power will go all the way to 27 dBm (501 mW).
I've also found that both radios always use the same country code, even though LuCI allows you to configure them separately. So for example, if you've got radio 1 set to US so that you can use 20 dBm and you change the country code of radio 0 to GB then it'll reduce the power of radio 1 to 18 dBm because it's also changed the country code for radio 1 despite the config and LuCI saying that it should still be US.