Seconded, they are all expensive (c.10% of the value per year) and stuffed with small print to get out of most claims.
The only viable option with any sort of value for money, and solid cover is to put them on a home insurance policy, but you still need to check the small print as some have approved locks etc.
(if all those years ago, mine had paid me for a stolen wheel instead of insisting it was an ‘accessory’, I would still advise against it but wouldn’t feel so strongly to shout about it everytime someone asks 🙂 How can a vital part of a bike be an accessory!)