Broadly speaking, the "bad" spot for a buffer is when the next device in the chain has a low input impedance that you want to exploit deliberately. Vintage fuzz pedals are the standard example: they have a relatively low input impedance, meaning they load guitar pickups; the result is that they are very sensitive to the guitar's volume knob (and the pickups themselves). If you have such a pedal that is particularly sensitive to your guitar's volume knob, it
could be due to having a low(ish) input impedance, and putting a buffer in-between could affect that.
Taking a quick look at the
Poison Apple schematic: the first part of that circuit is a buffer (Q1/2n5157). So I would expect that putting a C-Buffer in front of it would have no noticeable change in tone or feel.
As for overdrives: that's too broad a category. Some overdrives have integrated input buffers, like the Poison Apple does. The Tubescreamer is a classic example. But there are also overdrives that are similar to vintage fuzzes, in that they deliberately have a lowish input impedance so as to be very sensitive to pickups and guitar volume.
Also note, some commercial pedals have integrated buffers. This is true for Boss in particular: I'm sure there are some special cases, but essentially all of their compact pedals are buffered. So putting a C-buffer in front of a Boss pedal shouldn't affect anything (but would be redundant, since the Boss buffers are active even in bypass).
On my own pedalboard, I put an
always-on buffer as the very first device the guitar sees. I know none of my downstream devices change in tone or feel when presented a low impedance signal.
You could make the C-Buffer a standalone pedal, similar to what I did, but with a bypass switch. Then you can put it at the front of your chain, and go through each of your pedals one by one, with and without the buffer, to see if you notice any difference in feel or tone.
