Yes, very late 317's had an Onan. All 318's did. I don't recall them having a lot of valve seat issues though. Nothing like the B&S 16hp that was in the 582 (Cub Cadet). Every one of those dropped valve seats I think. Besides, Deere used the B43G and Cub used the B48G.

316s too. I'm not super familiar with Deere problems, but I haven't heard of any major issues with the Onans in them (or the B48G in the 982 either). Mostly it seems that people love them, they are just expensive to rebuild. Some quick googling seems to confirm that Onan had some intake valve seat issues with the performer line in certain years, usually associated with poor maintenance (as most engine issues tend to be).

I also haven't tinkered with the HTwin Briggs engines, but have heard anecdotally that they have leaky intake manifolds and that carbs are super expensive. Wouldn't be scared of one, but if I also would be inclined to repower a 582/582S if I had engine trouble with one.

Leaking intake manifold was a problem with both the Onan and the Broke and Scattered engines.

All the repowers I have done on the JD's is an Onan giving up between 1200 and 1500hrs... throwing rods....

I put new pistons and did a valve job on a M18 a couple years back. Used aftermarket pistons, had to learn how to fit rings to the bore, I did not bore and go .010" over as the bore measured fine (just honed the bore)---for clarity I measure things for a living and was not using measuring tools from Harbor Freight. I lapped one head on a surface plate, the other I had to mill a little off or sand for half a day. We use this mower a lot, more than any of the others. I don't think I have added any oil to it all season.

My 2072 has around 1,000 hrs on its original engine (yes I KNOW its original).

My 982 has an Onan. Its the Harley of lawnmower engines, gotta love that sound! Seems to have more torque than the 2072, but its not like I load either of them enough to choke them down. Parts are harder to find and there will be no apologies for high prices. Were it to blow up, I would decide then if I should fix or replace--after all it is the "Holy Grail" of Cub Cadets in many collectors eyes.

Got a 782 with series II. It is for spraying Roundup. If it blows, I'll find another cheap engine or cheap lawnmower to do its job.

Myself, I like the twins.

I have a tiny bit of experience with twin cylinders, I had a Briggs and scrap iron 18hp opposed twin that was never consistent. It hated any weather. Had oil issues associated with fuel going straight through the carb and into the cylinders, fuel pump issues, and all that crap. It was a 1995.

The 1974 1250 has had (almost) no issues so far with the engine, save for a few close shaves and almost stalling from lack of fuel. It's almost 45 years old and more powerful tan something that wasn't even 25!

I have a friend who has had not one, not two, but THREE onans catastrophically fail on him. He properly serviced them, too.

Sounds like your briggs needed a carb rebuild and maybe a new fuel pump. Probably nothing major. Maybe 40 bucks worth of parts. Very very common problems on almost any small engine.

I can assure you that a K301 would get walked all over by a properly running 18HP briggs horizontal twin.

Pretty much every engine will catastrophically fail if you run it long enough. Onans were premium engines when they were new (as were most Kohler engines). Most folks that have them love them, but pretty much all small engines in garden tractor applications (including K series engines) have a 1200-2000 hour lifespan before you're going to need to overhaul them. In generator service, many will last longer (sometimes much longer) than that. In heavy duty abusive environments, many will last less (sometimes much less) than that.

I don't doubt your friend has had 3 onans poop their pants on him, but there isn't anything inherently wrong with their design that would cause it to fail more than any other twin in that class. He's either unlucky, abusing them, or not really properly servicing them.

Well they all did sit for a while, as every tractor around here does, so it's not really a surprise they blew up. His 982 blew up while mowing as a crank bearing exploded, and the crankshaft spinning at 3600 RPMs broke a huge chunk out from around the rear hole, which caused a massive oil leak, and the keyway for the flywheel was destroyed, both on the flywheel and the crankshaft, his 318 overheated and dieseled full throttle until it threw a rod, and his 316 had scoring all up and down the cylinder walls and three or so massive cracks (1 in one side, 2 in the other) were causing the engine to completely lose compression, and at that point he couldn't really rebuild as the the whole tractor was also seriously abused and damaged.
He threw the 316 and 318 out because it was more trouble than it was worth to fix them, and he had other projects, and the 982 was probably repairable but he had a rebuilt kubota diesel from a recently rolled 782D that was totalled to put in, so he wanted the liquid cooling. It seems like every twin cylinder he's owned has been damaged or destroyed.
He also has had an original break in half on him. I guess he's just unlucky.

Edit: He rebuilds most of the engines in tractors he gets, but he didn't rebuild the onans and a few other engines.

That doesn't sound "unlucky", it sounds abusive. Someone that knows onans would have to tell me what sort of bearing that is, but "crank bearings" don't just explode for no reason. They seize due to lack of lubrication.

Engines don't overheat enough to diesel unless there's some sort of lack of maintenance issue (usually plugged cooling fins and lack of oil).

Again, scoring up and down cylinder walls and cracked blocks are not some sort of inherent flaw. That's just raw abuse.

Proper maintenance means keeping them clean. Keeping clean oil in them. Adjusting the carb and ignition system when (if) needed. Checking/cleaning/changing filters on a regular basis and checking the clearances on things like valves and such if necessary.

Yanking an engine that's been sitting for years, getting it to run, swapping the oil out and driving it hard is bound to give you problems.

Here is what a crank bearing looks like on an Onan B/P series. I'm not sure what can explode on one.
Attachment 95899

Onan did have an issue with the P series valve seats working loose in the late 80's early 90's engines. The Onan B48G's (original to the 982's)didn't have this issue. Here is one from my 1990 P220(out of a Grasshopper mower) that I rebuilt for one of my 982's.
Attachment 95900

The ones I've seen with a thrown rod have been the on the left side (opposite of the oil filter). That is the rod that gets oil last so I just write that up as being run low on oil.