I would think popularity too, not just how many were once sold, but how many are still in use today, not that I have any stats to back it up. Bottom line though is all tractors are still dangerous - high clearances, center of gravity, PTO, overloaded buckets etc. Old tractors even more so - no roll bars, seat belt, safety cut off switches, weak brakes etc. Respect the terrain and take it slow is the one lesson I learned. Especially working slopes, picking the appropriate machine for the job, keeping an eye out for large rocks and logs, etc. Common sense goes a long way, but things can turn deadly in an instant so stay situationally aware, including of people, vehicles and things around you.