Yup, you gotta be careful. If it feels wrong, don't do it. If you feel someone who wants to spar you has no control, politely decline and take a break. I declined someone a couple of weeks ago who wanted to spar tachi waza. Nice guy, but big. If he did a makikomi (or just plain fell) on me, I'd be screwed. I said as much, and took a break. My newaza is good for my belt level, so I have no fear of him there except getting dominated, but standing sparring is another thing all together. There is simply so much that can go wrong doing tachi waza randori that I now choose my partners VERY carefully. Fortunately, there are about half a dozen guys at my dojo that are very safe to spar with. We can relax and try stuff and have a good time learning with no ego. Randori should be fun, not terrifying. There are a few guys that I will not spar standing under any circumstances, as I'd be just spending my time trying to not get seriously hurt. I have no interest in perfecting my breakfalls at 100 mph thank you.