QCA9984/9994 for now is absolutely stable with CT driver and classic firmware, if you want stability in OpenWrt — i think it's better, because drivers is finely tuned a years on OpenWrt. But i have an Xiaomi R3P with same MTN7615 and it works as AP more than 1 year 24/7 without any reboots (no hangs or lags), all modern devices properly connected (as robot vacuum cleaner, smart devices, tabs, phones, PC's, notebooks), i think that is very good wireless chip with good potentiality and absolutely not worse, than QCA9984, but more cheap and cold.
And one more, i seen an dual band switching card version on MT7615, you can choose witch frequency run module. or run for VHT80 2x2 5 GHz + 2x2 2.4 GHz, it's very good advantage over one band QCA chips.