The coach doesn't seem to know the difference between trail running and running

Don't even look at the coach suggestions if your main activity is trail-running. Garmin in general does not even have a good implementation to consider elevation gain into all the metrics (this might be a bit better in the FR970 with the impact load and step loss feature). But the coach is probably just a copy-paste of the road algorithms and also does not work if you train for ultra-marathons.

Plus if you do not create your specific heart zones, Garmin will guess very wild ones and will suggest to you very inaccurate kind of training.

I would use the Garmin coach only as a start for beginners to train for short runs.

I do train for ultras... But in my case they are the Ultratrek like events, 20+ miles with 20+ lbs on your back on wooded/hilly/mountain trails.  For that I just track using the hiking function and the heart rate zones I created.  I use trail, and road, running to help build my base and LT threshold for those kind of events.  I love it on the road side because it forces me to do base and recovery says.  I am horrible at not treating every run like I am doing my 5k race pace.  Once I started to integrate trail running into the routine I was hoping for a similar experience but obviously that wasn't happening.

Thanks for the input regarding the coach.  Maybe I will invest in Training Peaks again.

Yeah I wish also for a better implementation. Without a proper elevation/descent algorithm, the coach is very unstable in planning workouts.

I understand that one cannot include the surface condition in an algorithm but elevation gain should be feasible. I guess trail running was not the main target of Garmin in the past but this sport is exploding in popularity. Sooner or later they have to close the gap here.

Yes it doesn't take into account the elevation even if you load an event track. The race finish time will be based on a flat course and the coach will no recommend any kind of hill training. 

What I do is most of base training planned by the Garmin Coach, I do it on trails using HR instead of pace. I might also do some of the sprints on a short hill. I have been training this way for a year. It helps that I've been running on trails since 2013 and most of the time I trained without any guidance. I use the garmin plans just to give me some basic guidance. I occasionally run on road races and if I set an high enough goal for a road race, mixed with my usual trail running training that works great for my trail races. 

Correct. Use HR based training thresholds for trails unless your trails are perfectly flat and groomed.