1a. The importance of environmental protection can't be exaggerated too much.
=> meaning, can be exaggerated, but not too much.
1b. The importance of environmental protection can't be overexaggerated.
=> meaning, can't be exaggerate.
2a. You can never be too careful in choosing friends.
=> meaning, being overly careful is the norm.
2b. You can never choose friends too carefully.
=> two meanings: 1) same as 2a, and 2) don't choose friends carefully.