They can be useful but do have limitations - especially as it isn't easy for a KH to be sure a wearer has no spare key. But if that can be secured, then OK in the right scenario.
I use emlalock -- I do set the the max or pick a session that has a max.. I don't trust to leave it open to random people
. Hence why it is up to the wearer to have a verification photo... with a tamper proof tag, though you can see how I trimmed the wires short, I discovered after a week the cables fatigue and break...so the excess needs to be tied down rather than cut off to stop the tag from flopping about, but once this is done, well the longest I have been "sealed" is 3.5 weeks. Please see my profile pics on how this tamper lock is applied, it should also work for the holy trainer. But you cant defeat it...even if you had 50 copies of keys made. I even tried once to get a copy of the key made for the looker...at the dyi store they just said ..."nup" we can't do that one...locksmith looked and me weirdly and said...."thats a german security key, I might be able to order the blanks if you really want".....needless to say I didn't Also for emlalock I use a electronic safe, a cheap one at DYI cost about $30 and has a 10 digit combination. Rather than take a picture of a combination lock we put keys in the safe. I use a random number generator to generate a text file full of numbers...then program the lock using the text file on whatever happens to be on line 6. If one does not intentionally commit a 10 digit number to memory its pretty hard to recall...then instead of encrypting a picture you just encrypt the text file. In my case Mistress programs the safe so these is no chance i cam memorise it anyway...but I have done it before and I really cant remember 10 digit sequence unless I "learn it" and punching the sequence into a safe twice is not enough to learn it. The safe is stong enough to survive many blows with an axe, but if something really goes wrong...it could be opened with a lot of noise and trouble not to mention the cost of $30