First, an enthusiastic +1 to Steve's comments re security being
compatible with risk.
Second, Mailman is -- at this point -- easily the best available option
for mailing list management. That is not to say it's perfect, of course
it's not -- but in terms of capability, support, development, community,
standards compliance, etc., it's the best we've got.
Third, if the data path from the IETF Mailman instance to your mail
client is compromised, then there are far more serious problems to
worry about than someone spuriously unsubscribing you or switching
you to/from digest mode or similar.
---rsk