Downsides?

What, if any, are the downsides of using a low temp solder for something like track feeders?  Are the connections lower strength?

Solder hardness and temp

I’ve not noticed a solder hardness issue with low temp solder. If anything, some of the lower temp solder like Tix is actually *harder* than regular tin/lead solder -- more like silver solder without the high temp. Tix is great with trackwork, such as for soldering points to a PC throwbar. But you do need to use a flux ... I have taken to using a no-clean paste flux.

Bottom line, it’s all good. The main difference is the price — you get less solder for the money once you get into low temp solder. Standard 60/40 tin/lead solder with a rosin core comes in big spools for an inexpensive price.

You do have to crank up your soldering iron temperature a bit with the low temp stuff. Basically, set it +100-200 °F or +50-100 °C above the solder melting point to get it to flow nicely and compensate for heat transfer loss.

With Carr's 70 (158 °F) or 100 (212 °F), you can still be under the melting point of styrene (205 °C / 400 °F) at +100-200 °F. I use +150 °F and it works fine. At that temp you can afford to linger on the feeder joint a bit and still not melt the ties.

Patience...

I've gotten seriously spoiled by the Amazon age, where you can sometimes have stuff on your doorstep the next morning, and usually never more than a few days later.

I've got a call in to International Hobbies to order some of the 70C solder, and am waiting for a call back since yesterday.  Perfectly reasonable for a small hobby business to take a few days to get back to me.  I have to keep telling myself to be patient!

Same deal with ordering some gap masters from American Tie and Timber.  Patience...

I'm eager to try some of these new things that weren't available last time I built a layout.

Yes patience

Other source, patience

I'll order some of the Carrs 70C when they get back to me.

I found a couple fairly low temp solders at digikey which I've ordered to try in the meantime:

118C:  https://www.digikey.com/en/products/detail/chip-quik-inc/SMDIN52SN48/8681827
138C:  https://www.digikey.com/en/products/detail/chip-quik-inc/SMD3SWLT-040-10G/12090475

My very first mail order was in the 70's, for two Troller autopulse walkaround throttles.  Got the info from an ad in MR, wrote a letter saying what I wanted with a check enclosed, and 6-8 weeks later I had my throttles.  That was some anticipation!

And just for the record

I heard back from Wayne at American Tie and Timber within an hour or so of sending the email, and as of now my order is shipped.  That's good service!