Mylar blankets are not a complete replacement for a tent or sleeping bag. If you're going on a backpacking or camping trip, bring a good tent and sleeping bag, don't just rely on a mylar blanket. While a Mylar blanket can keep you warm, it doesn't trap heat as effectively as a sleeping bag. If this is your only option, a Mylar blanket will keep you safe until help arrives. Additionally, mylar blankets are not padded and do not provide much comfort for sleeping.
Likewise, while a tent is windproof and waterproof, it will always be more effective than a blanket. The tent can handle changes in wind direction while also keeping you safe from wet ground. If you are going to be rescued within a reasonable amount of time, using a Mylar blanket is a good choice because you can save setup time and get warm quickly while still keeping an eye on the rescue team.
Emergency blankets are very useful and versatile tools, especially those made from Mylar. They are the perfect addition to any medical kit or survival gear. Being prepared is always a good idea and can greatly improve your chances of survival in an emergency.