I totally get the appeal of high-temp ice and water shield, especially with all-day sun exposure. But I actually went with a synthetic underlayment last year on my place (asphalt shingles, midwest winters), and it’s held up surprisingly well so far—no curling or leaks, even after some wild temperature swings. It was lighter and way easier to install, which mattered since I was doing most of it myself. Not sure if it’ll last as long as the pricier stuff, but for me, the trade-off in cost and effort made sense. Maybe it’s just luck or our less intense sun, but I’m curious if others have found synthetics to be a decent middle ground.
I’ve seen a lot of synthetic underlayments hold up well for a few years, but I’ve also inspected some roofs where they started to break down faster than expected—especially around fasteners or if there was any ponding. High-temp ice and water shield is definitely overkill for some climates, but in spots with a lot of freeze/thaw or sun, it really does seem to outlast the synthetics. The lighter weight is nice for DIY, though... just depends how long you want it to last before another round of repairs.