For a "value" generator head the ST's are worth considering. The ST's have some well known issues (my ST-3 had bearing and diode bridge/doghouse failures within 100 hours of operation) as 38AC states. Some vendors may have addressed these for you.
In my opinion they need an AVR added, driven from the mains not the harmonic winding, which is easy and relative cheap to do. Then the output waveform is acceptable for most applications and most solenoids and motors won't growl or buzz. The degree of "ratcheting" and spiking still present on the AC output does vary between units/vendors quite a bit. I have seen some o 'scope shots sent to me by guys with ST heads working on my homebrew AVR design that were pretty nasty. Microprocessor controlled battery chargers or PV type charge regulators sometimes refuse to operate with ST outputs without some effort to clean up the spikes and EMI. (Motor run caps on the output and sometimes metal film caps on the bridge.)
You will have to do regular slip ring/brush service, which is easy. Virtually no problems or reliability issues outside of the known issues have been reported, probably due to our best vendors/importers doing a good job of weeding out the worst ST makers years ago. Utterpower reported on the known issues and on some of the flakey, poorly built lightweight ST clones a decade ago.
If cost isn't an issue, there are certainly better quality, brushless generator heads, with skewed windings to avoid steps and spikes on the AC waveform, built in AVR, good bearings, and overall much better quality control. Marathon is one of the best, Mecc Alte (sp?) has also had some favorable reviews, and there are some others though my memory fails me on names. I've seen o'scope shots of Marathon waveforms that are such lovely clean sine waves they make me envious, and others that were a big improvement over the STs. Permanent magnet generators are an interesting option for those that loathe any electronics. (Though flicker will be comparable to a non-AVR ST head.)
ST heads are like an old Volkswagen bug- noisy, not the best ride, but they'll get you there, usually. Some people want a better engineered, quiet, smooth vehicle and are willing to pay for it. No fools, either way.