Upright drum smokers are good cookers. I have cooked on a Big Poppa Smoker kit drum for years. Quality of food is good. You can set temperature and it will hold it for a long time. The Horizon looks like a better drum cooker than my BPS, mainly due to the fact that the vent tubes have the controls higher instead of having to get down to near floor level to adjust air intake vents.
That said, you lost me at the statement "I don't know why anybody would buy a pellet grill" or "this thing whips a pellet grill hands down". Just silly statements.
1. Even while drum cookers are easy to use. Pellet grills are still way more easy and way more convenient. Push a button and turn a knob to desired temperature. Nothing easier than that. With a drum you have to light charcoal. That means you have to disassemble your cooker to get down to your charcoal basket which is at the bottom of a 55 gallon drum. A bit awkward. I had a hook that I could grab mine and pull out to light. Or dump a chimney on top of the coal basket. Not a big deal but a pellet grill is still way easier. Oh, and many pellet grills, mine included and be started remotely with a phone app. It is really cool to start the grill from the office or the store and have it up to temp ready to roll when I get home. does your horizon do that? guess not.
2. Once the charcoal is lit, you have to adjust the vents on the way up to insure a locked in temperature. Anybody who has ever run a kamado or drum knows that the best way to do this to gradually close off the intake vents until you hit the right temp. You just don't start shutting down when the temp hits 275 or whatever your desired temp may be. This would take me an hour or so babysitting this pit to make sure I got it right. If you get it wrong, you overshoot your temp and it is a long process to get it back down. Once again, not a huge deal, but well beyond the simplicity of a pellet grill. With my pellet grill, I can hit a button, throw some meat on and I am out the door. No adjustments needed.
3. Did I mention overshooting a temperature? Of yeah. Good luck getting your horizon down very soon if you have overshot your temp. Once again, the horizon is a much better looking drum than my drum but in my experience, once you overshoot your temp, just ride with it and don't expect to get it down much. That also goes for a kamado. With a pellet grill, if you are cooking at 275 and decide you want to bring it back down to 225 for whatever reason? No problemo and you can get this done reasonably quick.
4. Food quality? all subjective. I preferred flavor on my drum to the kamado to be honest. But I like the cleaner taste of burning pellets too. I actually prefer ribs cooked on my pellet cooker. Of course, the best tasting food comes off of stick burners, and pellet cookers, kamados and drums are always chasing that standard. It is all a tradeoff, but they all have their place.
5. Quality of combustion: Cooking with charcoal, especially lump can be a bit inconsistent. Reasonably close, but you have some days where you are burning cleaner than others. With pellets, you get the exact same combustion every time. You will get stronger smoke at different temperatures and a little more intensity depending on pellets used. But when I burn cookinpellets 100% hickory at 250, I know exactly what I am going to get with no surprises.
Plenty reasons to own a pellet grill as there are plenty good reasons to own just about any other quality cooker, whether it is a drum, kamado or whatever else.
