When jobs go sideways after they are underway, there is usually one step missing, and it is the step contractors cut first. Preliminary engineering is a scoping phase done before the main work gets priced, where an engineer confirms what the problem actually is and what fixing it properly will take. The point is to make the real scope visible before anybody spends money against a guess.
What you're actually choosing between
Bid from the client's description
What it is. You price the job on what the client says they need and start work.
The trade-off. You save a line item and a few days at the front, and those savings are real enough. They are also small. Expanded scope, the wrong equipment, schedule slips, commissioning trouble: most of it goes back to nobody paying to define the problem before somebody designed a solution to it.
Scope it informally during the bid
What it is. You walk the site, ask better questions, and adjust the number based on what you see.
The trade-off. It is free and it catches the visible problems. What it misses are the ones that need analysis, which are the ones that cost real money. Available fault current, coordination requirements, equipment compatibility and lead times only turn up once somebody models the system.
Run a preliminary engineering phase
What it is. A priced, defined phase where an engineer confirms the true scope, the right equipment, the lead times, and the order of operations before the main number is final.
The trade-off. A real cost and a real delay at the front, usually a small fraction of the project. What you get for it is the whole picture before the bid is locked, so the number is right, the equipment is right the first time, and commissioning is not a scramble.
You're moving the cost, not avoiding it
Skipping it doesn't save you the cost. It moves the cost to the point in the project where corrections are most expensive. The information turns up either way. What changes is whether it arrives while you can still plan around it, or after the number is locked and the wrong equipment is on a truck.
There is a client side to this as well. When real scoping is part of how you work, your clients stop getting hit with change orders and the number you gave them holds. A reputation for clean, predictable jobs is worth a great deal more over a few years than the savings from cutting the step.
So when you are comparing engineering proposals, look for the one that defines the scope before it commits you to a price, rather than the one that is cheapest at the top.