The stimulus checks aren't a test of UBI in part because we were told they were one time relief for an emergency, not an entitlement you can count on forever. People spend money differently if they are told "it's a one-time gift" versus "you can expect this for the rest of your life."
Historically, when you inject money into a system without increasing availability of goods and services, the result is inflation. If you start giving people UBI as a regular thing, the value of those checks will promptly go down due to inflation.
All welfare programs have a long history of failing to keep up with inflation. Even food stamps tend to last only about three weeks out of the month. I see no reason to believe UBI would somehow magically escape this pattern.
Making college loans readily available didn't fix things. It didn't mean that everyone had equal access to a college degree and now ordinary people could readily pursue the career of their dreams. Instead, it resulted in tuition skyrocketing and students having trouble getting a professional job with which to pay off their student loans, so people are waiting tables, putting off marriage, putting off homeownership, putting of having kids, putting off their lives to try to pay their loans.
I see no reason to think UBI would somehow go differently. I read that they tried something somewhere and the result was the landlords just raised rent and it didn't provide real relief for poor people. It just enriched the landlords.
Two-thirds of lottery winners are bankrupt within five years and they are at dramatically increased risk of being murdered, among myriad other terrible fates. The money doesn't solve their problems. They trade poor people problems for rich people problems and they don't have rich people coping skills. It often has very ugly results.
If, instead, you give people access to food and medical care and you make smaller homes in walkable neighborhoods where it's possible to live cheaply, then people can manage their problems and make their choices and find a path forward. People aren't likely to eat ten times what they need just because you are willing to pay for it. People aren't likely to get ten times as many x-rays and surgeries as they need just because you won't bill them for it.
Covering basic needs and guaranteeing you can eat and see a doctor is fundamentally different from cutting a check and telling people to spend it any way they want. It's an important form of social safety net that helps when things go wrong and isn't as readily abusable as cash is whether through innocent mistake, ignorance or even willful irresponsibility.
If you go nuts and spend like crazy and can still eat, well you might survive long enough to learn better. If you piss away all your UBI and then people say "Nope. No food stamps. We ended that program to fund UBI. Go starve." you've got a serious problem and so does society.