I'm in HomeKit hell too at the moment. It all started after Hue's Adaptive Lighting firmware update, which for me was the day after Thanksgiving. Eight weeks of automations randomly not firing. I share your frustration about lack of any kind of event logging. Because you have no error log to go look at, you spend hours and hours fiddling with things that have no impact on the problem. Timed automations run fairly well, but automations triggered (by a Hue button press, door sensor, another device turning on etc) are totally hit and miss. I mean it will work one minute, and 5 minutes later, it won't... or it will but after 30-40 seconds of delay/lag.
I've rebuilt the network, smoked all hubs and added them back in, deleted and re-created automations. Nothing has fixed it. Until Apple gets serious about HomeKit and provides users with feedback on failures, we're all doomed to a life of home automation unreliability.