The Thick Bertha is a very slow artillery. Its upgrading capabilities are gigantic, they even make fear.
But to do that, it should be made long-term management. Once your section reached a good level of experience, it will be necessary to spend much of efforts to protect this unit because the enemy will not think any more but of it!
Warnings with this difference with other artilleries: it cannot hold a fortification. It thus should be well protected.