It is not clear how pervasive the exhaust issue is and what models or years are at higher risk. You will hear more about 225s since there are a ton of them in service hence more reports of problems. I have read about the problems in 225s, 250s,lately a few 200s ( 6 cyl) and I think no 300s.
The more interesting cases are folks with twins who had the problem on one motor only! That is hard to explain unless the problem is not as much a change in alloys as it is a quality control one.
Personally, I would steer clear of any motors in the 04 through 07 model years unless there was an inspection that included removal of the power head and oil pan. and if you are going to do that you might as well install the new kit.
From the fairly decent amount of research I've done on the topic before, it sounds like other models of Yamaha outboards can and do have the issue at times as well. From looking at other forums like THT as well as others, some people with F150's have the dry exhaust corrosion problems, some have even said that they have them on their F115's. Whether that's actually 100% true or not or if they just heard it from their buddy? I have no idea. Some others claim that there are even a few people who have had 2008 model year Yamaha's that had/have the issue. From what I've gathered, it sounds like the most affected model were the F225's and from the years 2001 until roughly 2005 or 2006. As stated above, there are some that claim to have had the issue even up until the 2008 model year and some people claim to have had the problem with 115's on all the way up to 250's. I've not seen anyone claiming to have the issue with an F300 as of yet, but if there are issues with the other models of motors then I'm sure it's definitely possible as well. I personally had been seriously looking for a Grady White 228 Seafarer for just over a year before I found my boat and was even looking at them casually before then. It seemed like 9 out 10 GW 228 Seafarers in the early 2000's vintage had an F225 on it for a motor. Some of them had F200's and others had F250's standard from the factory, but most seem to come with an F225, unless it was repowered and/or switched out to another brand/size of motor. I looked at a handful of different 228 Seafarers and put offers down on a couple of them as well, of which IIRC 2 out of the roughly 5 boats that I was interested in had the dry exhaust corrosion issue. On one of the others, it seemed like the owner wasn't being honest with me and I passed on that boat before ever finding out if it had the dry exhaust corrosion issue, and the others supposedly didn't have it. Although again, I honestly don't have any proof of this and am just going off what they told me. Had I proceeded forward with wanting to buy the boats in question then I would've had a surveyor or mechanic drop the lower unit and inspect for the corrosion problems.
The boat that I ended up buying was actually traded in for this exact reason. The previous owner, and is also a member of this forum, went to start up the motor and had water shooting out from under the cowling. He took the boat to Tri State Marine in Maryland to have them look at it as they're a Grady White dealership and they told him he had a hole in the block and that he'd need a new motor. New outboard motors were/are still in short supply, especially last summer when this occurred, so the previous owner Chris traded in the boat for a new one. Tri State then acquired a used F225 with just under 1k hours on it, inspected it for the dry exhaust corrosion problem, of which it didn't have it, and then sold the boat to me. I bought the boat, trailered it back home to Seattle and when I got back I had the local GW dealer winterize the boat as well as drop the lower unit to make sure there wasn't a problem. They dropped the lower unit and said that the motor looked "perfect". It seems to very much be a roll of the dice as to whether you'll have the problem or not and I know for a fact that not every single F225 produced has the issue. I don't even think it's 50/50, even though it might seem like it when you're looking at a certain boat that has them as their outboard motor. If I was to guess as to what percentage of motors produced had/have the problem, I'm thinking it's probably somewhere in the 25-33% range. Although it's impossible to know for sure, but it is a very common issue that occurred for years and years over multiple different models of outboards and even though I don't know how many Yamaha F225's were produced over those years... I'm sure thousands and thousands of them, if not more, have had the problem. Before I had my Grady, my previous boat was a 21' Arima that had a Honda 130hp 4 stroke motor on it and they were known to have bad castings on their cylinder heads, but they had serial numbers for which ones were affected. Before I bought that boat I double checked with the serial number to make sure it wasn't one of the affected outboards and had no issues with that motor at all. I even bought and flipped another 21' Arima with the same motor but one that was said to have had the bad casting or serial number that was affected by the cylinder head problem yet I never had an issue with that motor either. I think it's just a roll of the dice as to whether you'll have an outboard with the issue or not, and especially with the dry exhaust corrosion problem from Yamaha... I don't think there's much you can do about it. I've heard of guys flushing their motors every time they go out but as we all know, it's not from a lack of flushing out your motor.