Some of the disadvantages of pay-per-mile are:

1. Unpredictable costs - With pay-per-mile, your monthly transportation costs can fluctuate significantly based on how much you drive. This can make budgeting and planning more difficult.

2. Disincentive to drive - The more you drive, the more you pay. This could discourage people from driving, which may not be practical for those who need a car for work or other essential trips.

3. Mileage tracking challenges - Accurately tracking mileage for billing purposes can be challenging, especially if you use your vehicle for both personal and business use.

4. Potential for abuse - There is a risk that odometers could be tampered with or mileage could be misreported to avoid higher charges.

5. Equity concerns - Those who need to drive more, such as commuters or people living in rural areas, may end up paying significantly more than those who drive less.

6. Technology requirements - Pay-per-mile models often require in-vehicle devices or smartphone apps to track mileage, which adds costs and complexity.

7. Lack of incentive for fuel efficiency - With a pay-per-mile model, there may be less motivation for drivers to choose fuel-efficient vehicles.

These are some of the potential drawbacks that have been identified with pay-per-mile pricing approaches. The significance of each factor can vary depending on individual circumstances and transportation needs.