I've noticed similar issues with most Zigbee bulbs, and the problem was not limited to Sengled. The issues I noticed sound similar to the above: on/off usually worked reliably, but trying to do a "Set Level" or "Set Color Temperature" (or both, as you might do with Hubitat scene activations) usually resulted in one or more commands getting missed. Sending what I wanted a second time, ideally after at least a small delay, improved reliability. Modifying the driver (it's one of the examples they published) to increase the delays by a couple hundred milliseconds or more (between its own Zigbee commands for these Hubitat commands) helped a bit but did not entirely eliminate it in all cases for me.
I got rid of the Sengled in my setup and returned to what I was doing before I tried this: using a Hue Bridge. ("This," by the way, was using a dedicated Zigbee-bulb-only Hubitat hub. If you're mixing bulbs with non-bulb Zigbee devices on the same network, that's possibly a recipe for problems, though it shouldn't be with Sengleds that don't repeat. Most of mine were Hue bulbs with a few Crees.) A Hue Bridge network works with most Zigbee bulbs except unfortunately (for you) Sengled and Sylvania/Osram as they are ZHA-only, and seems to be flawless in terms of how the bulbs respond to commands. Hubitat's Hue Bridge integration works well (the only disadvantage is the need for polling to bring in states from external modification), though I wrote my own to address what I consider a few shortcomings with theirs.
I do know staff are aware of issues with at least how some Zigbee bulbs seem to behave for UK users. My hope is that whatever they discover for those (whenever they get around to looking at that) will help us over here, too. I wouldn't be surprised if part of the problem was with the devices (one theory: Zigbee bulbs don't like receiving commands "too fast," perhaps for the same reasons ones that are repeaters make bad ones), but I also wonder if there's anything Hubitat could do to make this better (and to be clear, I'm not talking about their behavior as repeaters, just their behavior even when they're on their own network in terms of not consistently responding to all commands).