Surprising yourself is tricky. It is much easier for somebody else to surprise you. In a similar vein, I find it more interesting to admire other people's creations than my own. What I enjoy about my own creations is the process of creating them and the accomplishment of finishing them, on the occasions where I'm able to consider something to actually be complete. But I don't get very much satisfaction out of admiring them. Some, but not much. Since they're mine, they are familiar and uninteresting. The products of other people's different minds are what are interesting to observe.
So I figure Coyote wanted to empower Reynard to give him a greater ability to do things that would be interesting for Coyote to observe and/or participate in.
As an aside, this is why I believe we humans were given free will rather than perfection, which to me seem fairly mutually exclusive. Being flawed and in possession of free will, we are infinitely more interesting than we would be if we were perfect. Granted, we are also more self-destructive this way (both individually and as a society), but that's another story...
Perhaps that is why Coyote's gifts are flawed.
Regarding whether it is intentional, I doubt you can just lump them all into one category or the other. That would be boring. Reynard's gift was apparently unintentionally flawed, and I think Coyote's personality leads him to create that kind of gift in general. However I would find it odd if he never gave people intentionally flawed gifts as well - those are great for pranks. Consider the real-world example of the pen that shocks you when you press the button. Furthermore, he probably intentionally does not put much effort into attempting to make his gifts unflawed, which is different than intentionally giving them flaws - the former is more unpredictable. An artistic analogy is speed painting vs. carefully making an imperfect image.
Regarding truthfulness, opting not to (or even being incapable of) lying does not imply morals or even honor. Depending on the way a mind is wired, distruth could be a completely foreign concept (of course this doesn't apply to Coyote). In Coyote's case I'm inclined to believe he maintains truthfulness because it amuses him to do so. He probably enjoys the challenge of tricking people without lying, and also enjoys the possibility of being trapped by his own words. There's also the possibility that he really can lie and has just been tricking everybody into thinking otherwise this whole time.
(Of course I find that unlikely.)