Almost all my gear weight is over the rear wheel. It's not a problem at all. My position on the bike puts some of my body weight over the front wheel. On the first day, for the first kilometre or less, I notice the bike is a little sluggish. It takes more energy to move and more energy to stop, and the bike isn't quite as responsive as when it's unladen. After that, it feels comfortable, as if I've always ridden with a rear load.

I haven't done rides with only a large underseat bag, but I've done some overnight and short trips with a tent, sleeping bag and small sports bag strapped on top of the rear rack. That's a similar load to what you're taking. It's not a problem, but keep in mind your gear is going to be a little higher than with panniers. The advantage of carrying such a small load is that it teaches the discipline of light, minimalist touring, which is good for any cycle tourist to learn.