What are the disadvantages of bottom-up decentralised approaches?

Project budgets can be imposed by using bottom-up that is decentralised approaches for example project manager or project team sets the budget.

The disadvantages top-down decentralised approaches:

a. Budgetary 'slack' or 'padding' or 'buffering' for example over estimation of costs assigned by participants 'just in case' over spending does arise. This leads to incompetent allocation of finance

b. Financial inexperience of project management and team

c. Slower process to formulate a budget since greater consultation needed.