I can understand the benefits of zip off base layers, but at the same time, Kuiu has inferior merino than FL and that's maybe the reason why FL doesn't offer it... yet. If the temperature swings that much to where you need to take off your base layer bottoms, then why wear a base layer bottom at all? Just start moving and you'll warm up pretty quickly. Wear the puffy pants if you stop and start glassing. One of the intended purposes of merino is regulating body temps. FL's merino in the hunting world is second to none. Zip offs somewhat defeats that purpose. FL's pants, obsidians and corrugates, are designed to regulate temps as well. I come from a backcountry snowboarding background and I typically forgo base layer bottoms because I'm hiking and overheat. With that said, I see the benefits of it and there's no need for the "zip off" camp to get salty over it. If Kuiu's zip offs work for you, then stick with it. That's my 2 cents.
Once again, companies can talk about various textiles working over a wide range of temps, but I have found that I am frigid in the morning and sometimes after sunset in just my normal pants, but will be absolutely roasting when the sun is out in full force. Multiple times I have dealt with this on hunts in Texas, where temp swings are generally not as pronounced. Puffy pants would be too warm and would get torn to shit walking through green briar and past mesquite, in addition to being heavier and bulkier than a baselayer. I have to decide if I want to be super cold, or be comfortable in the morning and then trying to move during midday and getting swampy and inevitably end up standing in some field with both boot unlaced and my pants on the ground and that baselayer around my ankles.
Edit: I know I can easily buy the kuiu offering. I have a merino 210 hoody from them and find it very comfortable and durable, but with some flaws in fit. I would rather buy from a company that actively promotes and advocates for public land and access for all. FL and Sitka do that, I haven't seen it from Kuiu. I've been very very happy with everything I've bought from them, but would rather spend my money elsewhere at this time.