The only disadvantage that I can think of is that an application-level downlink may be postponed until the next uplink, so may need an additional uplink if the device really wants to know if its LoRa transmission was received. (At the same time that’s very much an advantage, hopefully making folks think twice before considering any downlink.)
Like for my basic data rate tester, it is nice to ensure the downlink is always sent immediately (and I’ve explicitly disabled retries there). But that’s testing only, of course, not a real life use case. (And it would be better to use LinkCheckReq for that, or I could just add the uplink counter to the application-level downlink if confirmed uplinks were not supported. And most often I do not use downlinks at all but just use 4G to peek into TTN Console instead.)