Crash cooling and lagering with the FTSs are totally possible, but the only caveat here is that the chill water source is the real key.
While in a "typical" situation where your ambient room temp is 80 degrees F and you are trying to keep an ale at say 68 degrees a small marine cooler/ice box full of ice water might last for 24-30 hours without needing to add any ice to keep the water cool, when doing very aggressive chilling regimes like crashing or lagering, then you are limited by the chill water source.
While it is possible to get the temperatures down initially the trick is holding them down for long periods with a basic ice box set up. Some of you may then ask, what is the advantage of the FTSs over just stuffing into a full on cooler or fridge and/or how can I rig up my FTSs to do crash cooling or lagering ?
The answer to the first question is if you are a multi-batch brewer and you only have one cooler/fridge , then you can't be crash cooling beer A that you are getting ready to keg while you have a beer (let's call it beer B) sitting right next to it that just started fermenting - both in the same cooler. So what some folks have already told us they plan to do is have say a keg or other vessel with a chilled water source in their cooler/freezer then plumb out the chill water line from in there out to the FTSs.
You could also do this out of a kegerator where you have kegs of beer and just toss in a chill water keg for instance. Some folks are of course also talking about plumbing in glycol. Combined with the ability to daisy chain the immersion chillers, well you can see that there are some cool possibilities here.
Just keep in mind that full on aggressive and extended time crash cooling is going to require a chill water source that is extremely cold (near freezing). So the typical set up with a camping / marine cooler full of ice isn't going to be ideal for that purpose and would take a lot of ice additions to even get there.
Keeping temps in the 40's or 50's for lagering is, of course, an easier proposition, but again chill water source will be critical as the temp of the chill water has to be at least 10 degrees F lower than your target temp in the fermenter to have any real efficiency. So if you are trying to hit say a 55 degree F temp in your beer, then you’d want the chill water source at 40-45 degrees F over an extended period of time.
Out of the box and with a simple setup like an ice chest / marine cooler full of ice the FTSs will do a marvelous job of maintaining target temps on ales without you having to mess with ice additions for anything more than like once a day. That alone is HUGE for a lot of you (and us!), we know.
Where you have an uber high outside ambient temp (a 100-degree garage) or where you want to get the beer super cold, then the typical setup with an ice cooler for your chill water source begins to show its limitations and other solutions are required.