Making part of a combo more difficult to interact with is always a concern. When that combo is already viable at the highest tiers of play and you make a part strictly better its a good chance its broken. You have at length explained that you don't play current magic so don't flaunt your ignorance when others tell you something is obviously a problem. I'm sure on your table a 2/1 with hexproof that makes your undying creature comeback with an additional +1/+1 counter it would never be a problem. But in the real world this would cause problems.Quote from ReapThaWhirlwind »lol at "it's broken to put hexproof on a 2/1 with an ability that's purely symbiotic".
Look, everyone knows you don't know competitive magic by your own admission. So here is some solid advice that isn't limited to magic. When you hear something unbelievable about something you know nothing about don't think "that's ridiculous" instead think "so that's what things are like there." For more important things than subsections of hobbies you have no intention of delving into a cursory look into the subject should prove the validity of the claim.