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There is a lot of UHMWPE rope on the market, often sold as "winch rope." This tends to be much lighter in weight compared to nylon of an equal strength, partly because it tends to be a much smaller diameter for an equal strength. I have heard that it is very slick, and therefore it is hard to tie a knot or hold a knot, but I cannot confirm that.

Let's just say that a normal nylon rope can be ascended with hardware if it is about 3/8ths inch diameter to 1/2 inch diameter. An equivalent UHMWPE rope might be only 3/16ths inch diameter for similar strength. Can that be ascended? Most of my ascender hardware will work with rope of 8-12mm, so I am thinking that UHMWPE is too small to be practical. It might possibly be OK for an emergency rappel rope as long as it is not too slick. But, if it can't be ascended, that may not help much.

I wouldn't risk it. The UHMWPE I've seen is thin braided rope that's way too slick to rappel on even with small-diameter rappel devices. I'd be extremely cautious about using ascenders on it. I doubt it resists that kind of abrasion very well, or handles well under shock loading. I know people who've climbed with ascenders on thin Spectra/Kevlar, but that's kernmantel rope with different handling characteristics.

Something to replace a winch cable or as a throw-line for swiftwater rescue - sure.

What is UHMWPE? Isn't this Spectra.

Ultra High Molecular Weight Polyethylene

I think that it is Spectra, or pretty close.

You're correct, Spectra and Dyneema are UHMWPE but are normally just called by their product names. In my previous reply I was thinking of UHMWPE in the braided configuration used for winch cables etc like this.

This article talks about pros and cons in climbing applications.

I knew that there had to be some catch. Otherwise, we would all be using UHMWPE ropes instead of nylon+polyester for rappelling and ascending.