Below is a quote from NEH Ch 45. The NEH I find online is version Jan 2007, so it is not nearly as current. They indicate here that anti-seep collars are acceptable for "low hazard dams" which likely is almost all of us. CPS 378 is the guideline that points us to, it's more current, and it does not allow anti-seep collars. Furthermore, the very next page in the NEH states that several properly constructed collars failed upon first filling. Just a guess, but they seem to have recently concluded that the risk of failure is just too high, even for a low hazard dam. Likely it's a function of hydralic pressure, soil type and compaction. As you point out, there are successful collars that have years in service.
If nobody here has a filter diaphragm (sand bed), then one wonders if the dam builders have sufficient background (or motivation) to change to a technology that is new, better, and likely more costly. They state collars were not allowed on "big dams" in 1985. They seem to be saying the same for little dams in 2013.
For many years, anti-seep collars were the standard
design approach used to block the flow of water at the
interface of the conduit and the backfill surrounding
the conduit for all embankments designed by most
design agencies. Based on knowledge gained during
the period of intensive embankment construction by
NRCS and other agencies in the 1960s through 1980s,
the use of anti-seep collars was reconsidered. Beginning
in the mid-1980s, anti-seep collars were eliminated
in designs of major embankment projects because
they were judged to be ineffective in preventing many
types of failures observed. All of the major embankment
design agencies, such as the U.S. Army Corps
of Engineers (USACE), Bureau of Reclamation, and
NRCS, as well as private consultants, now specify filter
diaphragms rather than anti-seep collars. Filter diaphragms
have been recognized as superior to anti-seep
collars as a seepage control measure. The NRCS still
allows the use of anti-seep collars for seepage control
along conduits for low hazard dams that are built according
to criteria in Conservation Practice Standard
(CPS) 378. Filter diaphragms are required design elements
in embankments that are outside of CPS 378.
