Whether the engine is good or bad is not really the question....the question should be...will it do what it is intended to do? Different grades of equipment, like any tool, is built with a market in mind. Many people (most it seems) don't need a commercial quality tool (or tractor) but only want the convenience of a riding mower, with the look of a tractor so they can mow their 3/4 acre yard quickly. They don't want to spend $5k plus on a tractor with power steering, hydraulic lift etc and a 2000 plus TBO engine. So, appropriately, equipment manufacturers recognize that market segment and produce a lighter built and less expensive alternative so they can have the mower they need. It really all boils down to...using the correct tool for the job. When the incorrect tool is chosen, that is where trouble begins. I don't think any of the manufacturers purposely build equipment which is sub-standard. They want repeat customers. If you look at JD ads...they market their equipment based upon how it will be used, making it clear up front what the intended purpose and limits are of their equipment.
Kohler has been a reputable company, like Briggs and Stratton and others and has been around a long time. Their engines are tools and if used and maintained as intended, will last a long time. Mostly it falls upon the consumer who uses the product.