It definitely works better in lower humidity, but as stated above, it works even during high humidity. What I’ve found is that it is better to get water temps low at the start of the season and keep them that way. That’s partly because I want to be able to jump in the pool at any time and not have to wait a day or 2 to cool it down, but also because once the weather gets really hot and humid, it takes longer to get the pool back to where I want it. My pool hasn’t been above 83° all summer this year, and we’ve had temps of 100+ and humidity above 80%. Our humidity typically drops a little during the day, as the sun burns it off, so I run the chiller from 3am - 8pm. 3am is usually when temp hits its lowest point. With that schedule, my pool stays in a range of high-70s to low-80s. If humidity drops lower than normal, it will range from low-70s to high-70s. Higher than normal humidity hasn’t changed the upper end of the range, probably because there is normally cloud cover and lower air temps during those conditions.
In regards to the comments above about working in the Houston area, there are a number of Houston-area members here who own Glaciers and have reported success, with 8-10° drops in water temp. Even 3-5° is significant and noticeable. As far as the heat pump goes, I did quite a bit of research, because I also was concerned about the effectiveness of the Glacier. A heat pump will cost you more upfront and when run in chiller mode it’s VERY expensive to operate - basically running an AC unit with no insulation and no roof. At least, that’s what I found when doing the research. I don’t have first-hand experience with one.
My equipment pad is also in full shade all day, so my chiller is probably more efficient because of that. At least that’s what my installer told me. My automation only consists of being able to program on/off times. Temp control isn’t a programmable item, but the chiller really doesn’t work that way, as you can’t set a temp and expect the chiller to reach it. I added mine several years after the original pool build, so it does not have a dedicated return line, but still works “swimmingly”!
Best of luck in whichever decision you make!