We had that lurching with our commercial large mower too. The fuel additive worked for about a day, then it stopped working. I can't remember what the real issue was, but it had to go to the shop.
We also only use non-alcohol gas in our smaller engines. I also run it in my truck. I get no decent mileage on it with ethanol added fuels.
I don't expect fuel additives would fix all engine issues. Normally, I used Sea Foam in my gas, but with my surging issue on the riding mower, the Sea Foam had no effect. I tried some other additives with equal disappointment. However, the Star Tron I used on my riding mower cleaned the fuel system and carb. It worked on this surging issue whereas the other additives did not. I think I had to pay about $10 for the bottle of Star Tron, but in the end it saved me a $100+ repair bill to have the carb removed, cleaned, and put back on.
I have had carbs fail, mechanically, and needed to be replaced. So no fuel additive would have helped those issues. That happens. But the guys at the repair shop tell me that about 90% of their repair jobs are fuel related due to the poor fuel we have these days. Also, the new EPA certified carbs we have are much more prone to clogging, or gumming up, then older carbs. At least, that is what I was told.
At one time, I was so discouraged with my gas riding mower that I was seriously shopping around for a battery operated riding mower. I just wanted to be done with the whole bad fuel, gummed up carb, not running right gas motors. However, I read too many complaints that the batteries in those riding mowers die/degrade after a few years and the problem then is getting replacement batteries. Evidently, those batteries are almost always out of stock and people were waiting for months on end for replacement batteries. I guess every manufacture has propriety batteries and they are not interchangeable. I hope they start to standardize those batteries, but somehow when I look at the different tool lines with propriety batteries, I suspect they are not in any hurry to standardize riding mower batteries either.
I have an older truck which just developed a gas line leak somewhere hidden up in a belly panel. First clue, 10 mpg on the last fill whereas I used to get about 18 mpg. I have talked to a few mechanics and it sounds like it might be one of those repairs that would cost more than the value of my old truck. Need to find someone who will take a good look at the gas leak when the truck is up on a hoist and gave me a firm estimate.
I don't use the old truck very much, but it is the vehicle I use to hitch up the trailer and head on out to the county landfill to get all those free wood chips for the coop bedding.