It seems, to me, that with thick, well insulated walls,  the typical "frost free" garden hose connector (usually located in the band joist) is a nasty thermal bridge and may not work as intended.   If plastic pluming is used, how problematic is freezing?  Is there a better alternative way of providing for seasonal outdoor water uses?

I have ICF walls with plastic pipe, and yes my one hose bib did freeze. I found a longer one and got it far back into conditioned space, but, as you note, it will always be a bridge, but at least it doesn't freeze. I used yard hydrants for the remaining connections, and have had no problems with them. These I passed out under the foundation steps, so they go down below frost under the walls, then come back up. I couldn't do that with the bib as I had a slab in the way. If you do something like that be aware that code requires a check valve, and that the hydrant is to be marked with a non-potable water sign.

A metal frost-free sillcock is indeed a thermal bridge, but the inside area for heat conduction is quite small. Also, if you have two parallel paths, each of uniform R along the path, for heat conduction across the same temperature difference from inside to outside, one being the metal pipe of the sillcock and the other being the insulated wall cavity, then the steady-state temperature profile along each path will be the same and linear. The metal pipe will be near room temperature on the inside end, where the valve is. The difference will be that the rate of heat flow will be much higher along the metal pipe. If the pipe goes through the insulated band, then the R along the path adjacent to the pipe is not uniform, the temperature profile along that path will not be linear, and there will be some lateral heat flow in the vicinity of the pipe. Still, the temperature at the inside of the pipe will be near room temperature, so the valve won't freeze.

It is important to ensure a slight downward pitch to the pipe, from valve to the outside connection, so that when the valve is closed the pipe drains to the outside. The hose must be removed in winter when not in use, so that the pipe can drain.

Even an existing non-frost-free sillcock can be protected against freezing simply by wrapping it in flexible foam or even multiple layers of bubblewrap. If the foam is open-cell, slip a baggie over the foam-covered sillcock and secure it against the house with string. If the sillcock is fairly close to the ground, a sheet of anything (wide wood, cedar shingle, sheet metal or plastic) can be leaned up against the house to keep rain/snow off the wrapped sillcock. If the siding is clapboard or shingles, a small sheet of sheet metal can be slipped up under the row of siding just above the sillcock to serve as temporary flashing to keep the foam dry. With an exterior insulation layer over the very conductive metal sillcock, the temperature profile is shifted so as to keep the metal far warmer than outside air temperature. This is so if the piping to the sillcock is metal (copper), as the piping conducts heat out to the insulated sillcock to keep it warm. If PEX is connected to the sillcock, the insulation won't help much. For new construction, frost-free sillcocks are the way to go.

Is there a better alternative way of providing for seasonal outdoor water uses?

You could run the pipe underground and then use a frost proof standpipe/hydrant. While "taking it to the max" is always interesting, I would want to know what the actual $ loss is for the conventional, heat wasting design. Especially with one of those styrofoam cups on the outside (use the plastic covered ones, my pure styrofoam ones didn't last long).

I have a home with an enclosed crawl space.  Each hose bib has a cutoff valve inside the crawl space about 1' from the foundation wall.  Each winter I turn off the hose bibs from inside the crawl space and then open the hose bibs on the outside so the trapped water will drain.  Although my home has gate valves, I want my next new home to use ball valves.  Gate valves can be problematic and require several turns to open or close.  One-quarter of a turn will open or close a ball valve.

I have been told that water freezing in PEX lines will not hurt the line but may break the fittings.

You can improve the situation by simply giving some thought to the placement of the sillcocks like placing them around a corner from the prevailing winter winds, for example. Getting them out of that exposure can add some degrees to their temperature range.

Thank you all for the comments. To try to assess the thermal bridging's cost I assume the thermal path is equvalent to 1/2" copper tubing (5/8"OD, 1/2" ID). then I found conductivities for copper and oak in the same units copper is 2500 times as conductive as oak which I know to be r1/". From that ratio I conclude that that the copper pipe conducts the same amount of heat as 2 sq ft of oak. My walls are about r42 and I have about 2000 sq ft of them & they are about 14" thick so the effect of 4 frost free sill cocks is to reduce my r42 walls to r41.66. Simply not a big deal or price to pay for the convenience of frost free sill cocks. So long as they really don't freeze. From what's been said here I conclude two things are important in installing frost free sill cocks. 1 They must slope downward to the outside. 2. There inner end needs to be at room temperature meaning no insulation inside the pipe. Again thank you.

Well done Mr. Russell !

Two things to add: There is only one; Woodford Frost Free and kudos for finding a legitimate use for bubble-foil.

The terminal end of a frost-free hydrant (not to be confused with a sillcock or sillcock with hosebibb) needn't be at room temperature but generally in the conditioned space and above freezing. Making the terminal flange air-tight (we use fiberglass in consideration for the guy who will have to replace it some day), since infiltration is a magnitude above conduction when it comes to heat loss.

wo things are important in installing frost free sill cocks

I would add "they need to be long enough to pass completely through the wall".