Yes, most of those points are the concerns with warrant canaries. So far as we know the concept is totally untested in court so it’s hard to say what the result would be until it happens.
Updating the canary should require a human input (like a password to unlock the GPG key), which is not sometime the government would generally get access to (they make a request for data about XYZ user, and the company turns it over; they wouldn’t get actual access to the production system). The government could seek a ruling to force the company to update the canary, but as such a thing hasn’t been granted before (at least as far as we know), it’s not a guarantee. So, there is a chance that the warrant canary will serve to alert users to something happening, which is better than nothing. But because of its untested nature, it might be broken by a court.
I’m not sure I understand your point about “once it’s triggered it can’t be reset.” If a company fails to update their canary on schedule it means something happened that they can’t disclose. Once they are released from the NDA they can release a new canary explaining what happened.