I'm not a lawyer and this isn't legal advice.
Is there any science behind these tests?
If performed under correctly controlled circumstances by an unbiased, well-trained officer, they might be more likely than not to give the correct result. In reality, they are performed on the side of the road with flashing police lights, a nervous driver and a police officer that isn't going to waste their time administering them to someone they haven't already decided to arrest.
Are they just a blank check to arrest anyone?
An officer needs probable cause to arrest someone. An officer could have probable cause that a person is driving under the influence of a controlled substance without a field sobriety test, or could use the failed FST (according to the officer's administering of the test) as probable cause. It's possible that in some rare circumstance an officer has probable cause that someone is committing a DUI but decides that they were wrong after the person aced a FST, but that's not something that should be expected.
As far as I can tell, the relevant section in Oregon law for refusing a FST is ORS 183.136
If a person refuses or fails to submit to field sobriety tests as required by ORS 813.135 (Implied consent to field sobriety tests), evidence of the person’s refusal or failure to submit is admissible in any criminal or civil action or proceeding arising out of allegations that the person was driving while under the influence of intoxicants.
As you can see, the penalty for refusing a FST is just that the fact that you refused it can be presented as evidence against you, it doesn't carry a financial penalty or license suspension like refusing a chemical test. At least one attorney's office in Oregon (Romano Law) recommends that you almost never submit to field sobriety test (every other attorney's site I've found which makes a recommendation also recommends against taking a FST absent an attorney's advice for the particular situation):
Should I refuse a Field Sobriety Tests?
In most cases, yes. This is a complex area of DUI and search and seizure law, but in most cases, a police officer requesting Field Sobriety Tests has already made their mind up. You’re going to be arrested for DUI, and therefore they are only gathering more incriminating evidence of the crime of DUI with the FSTs. The FSTs are very difficult tests to perform without error, and anything you do incorrectly or poorly will be used against you to suggest you were drunk.
Ultimately, just about everything officers do during a traffic stop is intended to trick drivers into admitting to wrongdoing or provide evidence against themselves, including FSTs. Since you say you don't think you would pass them even if you are not under the influence of a controlled substance, it sounds like you may be better of not submitting to an FST (you are not required to explain why you are refusing the test either, and it is unlikely to be in your best interest to do so absent an attorney's advice), but if you are concerned you can contact a local attorney for a consultation.
Even if you are later exonerated through a blood test, should you really have to go through the arrest process just because you're a klutz?
The officer has likely already decided to arrest you before they ask to administer the FST, if it's any consolation.