Models built with no standard in mind may be poor or they may be excellent. There is no guarantee until an informed opinion is reached.
We work with clients in reviewing spreadsheets that do not follow the FAST standard and we help them decide the best course of action: continue to maintain a non-standardised model or invest to convert it into something that complies with the FAST standard.
Fixing broken models (where the design and construction decisions have led to a spreadsheet that is no longer fit for purpose) and replacing it with a FAST version invariably leads to spreadsheets that are easier to maintain and hand on to successive modellers and reviewers. Teams also find that new possibilities open up with a standardized model that they never thought achievable with the original model.