I had a neck dissection at the same time as my glossectomy. 7 months on, the scar is looking very faded and the lingering side effects are a region of numbness on my cheek and ear-lobe. Initally this was quite disturbing and uncomfortable at night since I usually slept on my side with that ear downwards. But it has improved steadily and now I have no discomfort really, just a strange sensation when I shave the rather sparse beard (following radiotherapy). The neck is stiff a little stiff and slightly rigid to touch but I think quite a lot of that is also the radiotherapy not just the neck dissection. As I'm sure they have told you there are risks of complications, particularly damage to the accessory nerve which can result in restricted movement of the shoulder. I had some physio which cured my initial problems and my shoulder is as good as before. Hope this makes you feel a little easier about the prospect.
Martin