They have three problems when used as carbon sinks:
1) They tie up land for one, otherwise, non productive purpose (the wild species living in them do not mind that, must be said)
2) Once fully grown they reach a carbon balance. This can happen surprisingly quickly, a few decades.
2) Forests do not simply sit there like a rock. Eventually they will burn unless managed. Then they simply release all that stored carbon into the atmosphere.
Regenerative agriculture is a much better carbon sink. The process of storing the carbon increases the soil fertility so it is a win/win. Soils can get very deep so there is not a natural limit like for forests.
A draw back is measuring the carbon sunk. I do not know of anybody doing that (a market opportunity?). An important part of carbon farming is selling the "carbon credits" (a mad bad and sad idea, but it is a thing) to do that there must be an accepted measure of carbon sequestration, like there is for trees.