Pretty effective app, thanks! There are two issues I've come across, maybe they are related.
The first is that I lose connectivity within 15 seconds to a minute when using the app on an LG Nexus 5X (Stock rom Android 8.0.0). The disconnection message I get is "Failed to connect to Sbricks.". Somewhat worryingly the Sbrick seems to still be connected as it continues to power the motors in whatever the last state was until I press the 'OK' button on the disconnection message box, then the Sbrick immediately stops what it was doing. The disconnection seems to happen only when i'm pressing buttons - if I just don't press any buttons on the gamepad (or just press a button once in a while) then it stays connected indefinitely. The faster I press buttons, the more frequent the disconnection happens. Spamming joysticks or triggers seems like a sure fire way to cause the issue. The problem occurs when using an I-pega PG-9021 gamepad (Bluetooth) or Xbox360 wireless controller (USBOTG), with Sbrick firmware versions 18 and 20. However I haven't experienced any disconnections when using the Ipega and Sbrick with firmware 20 on a Samsung Galaxy S2 (Cyanogen mod Android 4.4.4), so maybe its a problem related to Android 8, not having root, or the bluetooth hardware in the Nexus 5X.
The second issue also concerns pressing a lot of buttons or moving the joysticks back and forth many times. It seems like this generates a lot of commands to be sent to the Sbrick and they start to queue up, leading to severe lag as the Sbrick slowly works it's way through the queue of commands. I'm not sure if this is an issue with the app or the Sbrick hardware but it doesn't seem to happen when spamming sliders on the official Sbrick app. If it is the hardware then maybe the solution is to only issue commands as fast as the Sbrick can process them.
Here's a video that shows both issues happening: https://www.youtube.com/watch?v=si9zcAiBh1I