Are bad batches more likely to occur with the likes of Glenfiddich and Glenlivet?
Certainly not! Single malt whisky is very delicate as whiskies go, and they get most of the flavour from the cask. Occasionally a cask will be somewhat off or tainted, these casks will be vatted with other better casks and generally 'lost' in the vatting. What makes a whisky good or bad is more often than not - subjective. Perhaps a small rusty nail, or sulphur (from sterilised sherry casks), or a slightly rotten stave may effect the ageing whisky. Or maybe a non-compatible cask type (wine, sherry, or other).
Single malt is difficult to make and spirit occasionally leaves the stills in not the best shape, this is easily done, and it seems that some distilleries make a habit of it (but not Glenfiddich or Glenlivet).