Your concern is warranted (no pun intended).
But again, there will be so many people truly DUI this weekend & tonight the cops won't need to mess with the marginal cases. It becomes an even bigger waste of their time, resources & credibility than necessary. Why go through all that then have the perp/accused go free because your evidence wasn't strong enough?
Especially with so many sh*t-faced people on the roads this weekend?
Some jurisdictions seize the vehicles of DUI suspects, and force them to--even if acquitted--spend hundreds of dollars in fees to get their car back or else (for cars that aren't worth that much) just abandon their vehicles altogether. I don't know whether the cops here are in such a jurisdiction, but in lawless places that steal people's cars I can see plenty of reason to make DUI arrests without worrying too much about what will hold up in court.