The main strengths and weaknesses of this plan came from the same place. They were both related to the fact that Lincoln wanted to be lenient to the South.
Lincoln's plan was going to make it easy for Southern states to be returned to the Union. It said that any state could form a government as soon as 10% of the people who had been eligible to vote in 1860 signed an oath of loyalty to the US.
This was a great strength because it would have encouraged the South to reconcile with the North. It was a magnanimous gesture showing that Lincoln was a great man who did not hold grudges. But, at the same time, it could be perceived as a weakness. We do not know how things would have turned out had Lincoln lived. But Johnson's plan was similar to Lincoln's and the Southern states responded by doing things like instituting the Black Codes. This showed that they were willing to take advantage of the lenient terms of the plan to try to return to the way things had been before the war.
Thus, Lincoln's plan was great because it was forgiving and lenient. But these were also its major weaknesses.