The best short term health insurance plans often do not cover pre-existing conditions. You may also not be eligible to re-enroll automatically at the end of your term – you would be required to re-apply and go through underwriting again. A short term plan does not meet the minimum coverage requirements to be ACA compliant.
All of that being said, a short term medical plan can often save you a lot of money both in premiums and deductibles, and is likely more widely accepted than an ACA plan, allowing more freedom and flexibility in selecting your doctors.