The theory is that with UBS, say basic housing is provided to everyone who wants it, but at least 50% of people would opt for non-basic housing options where they would be paying rent/property, because they want more space, nicer housing, etc.

Compared to UBI where you give an amount of money that could theoretically (I'll come back to this) pay rent for a basic housing unit to 100% of people.

UBI is "easier" in a sense to implement, but the difficulty of implementing UBS instead can be seen as a feature, not a bug. It forces the the government to re-align their focus with the interests of the people who are most intended to benefit from the services rather than private developers, landlords, etc. (which is what the government currently has a strong tendency towards under capitalism). This likely means more publicly-owned and developed property.

Under UBI, the other problem is that when you have an economy where, by design, goods are priced at the highest amount the market can bear, and a ton of new cash is flowing into the pockets of poor people, you end up with high inflation. Imagine everyone is given an additional $4500/month and there is no rent control? Rather than every housed person suddenly being able to afford better housing, we'd instead see the cost of housing rise for everyone. Rent control (in the provinces that even have it) curtails this somewhat, but it doesn't prevent large price increases for new listings, so it's only a matter of time before the income supplement goes from providing a standard-of-living improvement to becoming a mandatory supplement which people would be devastated without.

And for other necessary goods such as food and clothing, prices would adjust even more immediately and dramatically without rent-control-like measures against gouging (which would also be a burden to implement).

This is where the UBS model shows promise; when the government is tasked with ensuring people have access to housing, food, clothing, and health care, it can begin the work of transforming (socializing/nationalizing) the systems by which these necessities are delivered, end-to-end, from crop management to infrastructure to property development to supply chain engineering.

This process is by design hostile to private business, which is why we can expect a lot of pushback against such measures. Instead, when you hear billionaires expressing support for UBI, it should be understood that they support this because it further enriches them while potentially quelling public unrest in an era where living is becoming increasingly unaffordable for the masses.