As households age, household sizes fall. And more and more homeowners are finding themselves in need of less house, and less yard. While it often serves as a 'nest' for 'boomarang' offspring (and their children), at some point, at some point the rental income from renting out a basement or an accessory dwelling unit becomes very attractive. Things like AirBnB only make this more tempting.
Even without permitting the construction of tiny houses as ADU's, you are going to get a lot of 'stealth duplexes' over time. They work out tolerable well while the owner is still resident*, but become problematic when the owner moves on, and an absentee landlord rents out both units (The problem is endemic in SLC). Regulations against this are difficult to enforce, as the change is often difficult to detect.
Long term, you are going to have the problem with 'stealth duplexes' whether you permit tiny houses as ADU's or not. Permitting them means people will actually pull permits, which means better conformity to building code for what gets built.
It also represents an opportunity for existing residents to 'age in place' without leaving the neighborhood. They could build a 'tiny house' ADU on a single story for an aged couple or widow/widower, and then rent out the main house for income.
Discussing them with Mike Malloy at SLC, he said the biggest issue they had run into was sewerage--where the units had to have their own connection to the mains, or could 'count' as part of the original dwelling unit--something the article in Ogden also mentions.
*Some places have elected to permit secondary units only where the owner is also resident. It does seem to help mitigate the problem. Violators (two tenants, absentee landlord) are legally vulnerable, which tends to cut down on problem tenants.