To answer your question, YES, the CBT is good enough to start with to build up "basic knowledge only".
However as your knowledge increase, you will soon realise the CBT is :
- too vague
- not enough detail
- does not answer all your questions
- does not have all the information you need
- might even have mistakes in it
- is generic only
- is old and out of date (perhaps the original CBT from over 20 years ago)
- and does not reflect the actual company aircraft, regarding updates/modifications/upgrades
Build up your basic knowledge through the CBT then move across to the FCOM.