I installed a new 5 in 1 on an oceanfront cliff 8/17 knowing full well it would be a harsh environment. It made it until last week and started giving whacky temp and humidity readings. I just completed an autopsy and this is what I found.
The 5 in 1 actually still looked like new inside. It looks like just about every metal part is stainless steel including the fasteners.
The problem turned out to be the SHT21 daughter board. It is completely corroded right at the SHT21 chip. It looks like the chip's leads were originally encased in a epoxy like sealant but it's complete gone on the bottom side. One SHT21 lead has pretty much completely disintegrated. I plan to swap in an HTU21D from Amazon and put the sensor back in service. I might block off that open port underneath the daughter board. It might slow down the corrosion. I'd think it would still give accurate readings but with slower response to changes.
In all fairness, I don't think a WS-2000, Atlas or Davis sensor would have done much better. Atlas and WS-2000 have ferrous mounting brackets so they might even be worse.
P.S. The sensor suite along side the 5 in 1 is a Maximum going on 15 years.