Look this just my own opinion based on my experiences but..
I had an emergency call system installed for my mother some time before she got "forgetful". She would wear the pendant intermittently (when she thought about it), but never had cause to use it. However, after her strokes and before I moved her into an extra care facility the damn thing caused nothing but trouble. If she pressed it accidentally, then disembodied voices would confuse the life out of her. The phone itself became more and more problematic and sometimes the activation of the call system would result in the phone line becoming tied up until I could get someone physically to go in and properly disconnect and reconnect it. It was awful.
When she moved to the extra care facility they had a built in call system - she never did remember how to use it, although she did use the call cord in the loo once. We were considering using the sensors that could be connected to the systems (someone has left their bed and hasn't returned for 15 minutes etc) when she fell and ended up in the nursing home (short form version).
My opinion - they can be great if there's someone available to sort them out immediately ( or within 15 minutes) but can cause more trouble than they are worth if not. Oh - and if they require any kind of learning curve on the part of the person, then forget about it. The irritating thing I found was that these pendants etc are available in other countries as waterproof watches etc - not in the UK. Once it's taken off then there's a good chance it won't get put back on, and as a pendant, it was way too easy to accidentally activate when she lay down in bed or something.