No 1st gen tank has a bad off road capability ( except T-95 ) but given the vids from the Lebanon conflict Merkava has some serious manouverability issues in difficult terrain.
As for the backdoor, given that virtually all damaged Merkavas got it up the ass and some were hit with rubbish like Spigots or RPG-9 AND mission killed which should not happen at all so yes its a major weakness.