A couple of comments about your unofficial poll and I am looking forward to the results you print.
1.) Congratulations on a job well done. Eight's on means you were flying about 1,000ft agl and you still put it down in your "safety zone". And, of course, now that you know that engines can fail - I'm thinking you will teach your students with just a little more emphasis on "engine out" procedures and picking safe sites for performing ground ref. manuevers (leave yourself an out!)
2.) I think your poll will be a little skewed towards a pretty ugly number - mainly because people will probably not answer you with a "no failures" answer. Don't let the results discourage or intimidate you. We fly a LOT of hours without failure. I personally own an airplane with a "bulletproof" O-360 Lycoming under the hood. The engine has run 4800 hrs and of the three owners in its life (I have 1800 hrs in it), this little plane has never let us down. However, as an owner, I take great pride in the plane and do some pretty regular maintenance.
3.) I think the engine question is legitimate. I remember seeing an article in Aviation Consumer magazine many years ago that reinforced my decision to buy a simple Lycoming like the O-360. My engine failure was in a Continental TSIO-360 with 800 SMOH. Engines which are high performance or are turbo-charged to boost their performance are more prone to failure (IMO).
4.) Life as a CFI will expose the pilot to far more airplanes and in more questionable condition than most pilots out there. As a private pilot after my initial training, I probably flew maybe 15-20 different airplanes. When I got my CFI rating and started working in that field, I could go through that many in a work week. At my airport, there are about 20 FBO planes, about 30 in different flying clubs and then another 200 owner airplanes. I have no idea who does the maintenance on all these planes, but I review the logbooks, do a pre-flight and go (if all passes muster).
Experience is a wonderful thing.