Having done a fair bit of orbiting, since I fly out of an airfield that has commercial operations, i would make the following comments.
You can have to orbit for a fair amount of time and you feel you may have been forgotton by the tower but you are not sure how to remind them you are still there or if you are allowed to.
Orbiting is very tireing and requires a high level of concentration, hard work in cases where there is only1 pilot.
In cases of the pre solo student orbiting is demoralising as they are not getting enough constant circuit experience to allow any but a brave instructor to get out and it also wastes the students money.
Extending downwind is not to bad unless you have to go miles further than usual, this is because as Beagle says the approach will not be of the usual type and the flare could be misjudged especially with students or inexperienced PPL's but we do have the option of a Go Arround if we dont feel comfortable or dont like the look of things. I will do anything I can to avoid comming in too close to a 737 etc especially as I am not sure what 4, 6 or 8 miles looks like.
Adjusting the circuit is no harder than flying to an airfield that is new to you, if anything it is easier since you are familiar with the landmarks. Some airfilds have very strange normal circuits that are extremly wide and if you do the type of circuit as taught ATC will ask you to move wider.