Fenns are very effective at catching. They are very easy to set, they are also very easy to set badly. Unfortunately they don't in my opinion kill quickly in a lot of cases. When set badly it increases the chances of a bad catch..... and as said they are easily and often set badly.
In the vast majority of cases there is a better option offering a quicker kill. Unfortunately these options often aren't as effective or easy or they take alot more thought to set. They are often a lot more expensive as well.
The law just stopping there use on stoats but not other things is badly thought o