That being said, with a drysuit you can still freeze your tail off if you don't have the proper undergarments. Just because your dry doesn't mean your going to be warm.
True. Here, temperatures vary a great deal throughout the year, from about 4 C (39 F) to about 18 C (64 F). Surface temperatures vary from -2 to 22. Personally, I think 15 C is nice in 7+7 mm, and at 20 C, a 7 mm will do. Still, I dive dry year round, and for that I use a trilaminate suit. It provides no insulation, so I put on undergarments appropriate for the water temperature.
For the coldest period, I use an inner layer of merino wool, 400 g thinsulate and a home made chest plate of insulating foam. Around 12-17 C I'll just use the 400 g thinsulate, and for the warmest month, I'll use 200 g thinsulate. I always use thick, technical socks. Of course, I also have an assortment of hoods and gloves.
Because neoprene suits compress and loose insulation, target depth should be taken into consideration. Air temp and weather also, because you want to be able to regain warmth after a freezing dive (which is of course harder if you're wet).
Too cold/unsafe is a subjective thing. If you're freezing, you're freezing, and that can in many cases be unsafe.