Is there any situation when Microsoft Dynamics 365 get failed

Question

We hired someone and he is well versed with Microsoft Dynamics 365 , although we have an ERP to build is this a good situation to chose Microsoft Dynamic 365 , 

need opinion or suggestion about 

When NOT to use Dynamics 365 

When Dynamic 365 Get failed

Is it provide full customization when used on larger ERP scale 

what its user limit  when we have 3000 users

whats its costing if i have 1000 users.