Oct. bow hunting for elk is definitely more difficult. You can get into very vocal bulls in Oct. and spotting them can also be easy. Neither of those are really the issue. 1. You have to be prepared for the weather. It can be much more difficult in Oct especially in MT. Not always, but you better factor it in. 2. The bulls have been hunted for a LONG time already. The better bulls are not going to be called in. You're going to be using more rifle hunting tactics, but with the range of a bow. 3. Often they are all ready or are getting very herded up. I've been in situations where I've had multiple accessible herds of elk in site, but they are so large and spread out, usually with the best bull(s) being near the center, that getting into range was impossible. Meaning I could be within 20-30 yards of the herd, but the bull(s) I wanted were 100+ yards away because of the herd size.
I actually like hunting elk in Oct with a bow because it fits my solo style of spot/hear and stalk, but I treat it as an additional season not my primary archery elk hunt. No way I would plan that time of year as my only hunt.