About 30 years ago, there was a lot of discussion about the process of converting the United States to metric units.
Proponents of this conversion debated whether the process should be called "metrification" or "metrication." They decided to call the process "metrication" on the grounds that there should be no "if" in the word! Well, 30 years later most Americans still haven’t made the switch (and don’t want to), so apparently the "if" would have been appropriate.
Current Situation
Now, most imported equipment is based on the metric system. Also, most major farm equipment companies have converted to metrics because they manufacture and sell equipment around the world. Smaller manufacturers that manufacture and sell primarily in the United States generally have not switched to metrics, except for certain specific components (generally imported) that are available no other way.
Tools
One effect of the current situation is that Americans need two sets of tools to maintain the equipment they own. Conventional wrenches sized in inches are needed for older equipment and for some current equipment manufactured here, and metric wrenches are needed for imported equipment and equipment from multinational U.S. companies. Some equipment has both inch and metric components, making it difficult to know which wrench to use.
Fasteners
A major problem we face in the United States is poor availability of metric fasteners. Many pieces of lawn and garden equipment use metric fasteners, but metric fasteners are much less readily available at hardware stores, home centers, etc. than are inch fasteners.
Even when they are available, metric fasteners tend to be much more expensive than inch fasteners. A recent visit to a farm supply store revealed a typical scenario: inch fasteners were available in bulk in three different strength grades for reasonable bulk (per pound) prices, and only limited sizes of metric fasteners were available and were priced singly (and expensively). Most farm equipment and lawn/garden equipment dealers do carry a good line of metric fasteners, but the metric fasteners are expensive.
Other Off-the-shelf Components
Many other small components on lawn and garden equipment besides fasteners can sometimes be purchased at hardware stores, home centers, auto parts stores, farm supply stores, etc. without having to visit a dealer. These components include O-rings, roller chains, belts, hydraulic and fuel line fittings, bearings and seals. Like fasteners, these components are much harder to find and more expensive if metric. You will be much more likely to have to purchase the parts from a dealer if the machine is metric.
It is not necessarily desirable or practical to try to avoid equipment designed and manufactured to metric dimensions, but be aware of the need for duplicate tools and more-expensive fasteners and other common parts.