Competency still applies. And regardless of property type, the benchmark for that competency will be based on what appraisers do who normally appraise such properties. Whether its a house or a condo or vacant land or religious or commercial or industrial. The measure of competency remains the same.
Mixed use can get real dicey if there is a lack of really comparable sales data. Starting with the point that the contributory value for two different uses onsite can be and often are unequal. You probably will have to run two separate rental surveys and possibly two different sets of comparables.
Now an SFR + salon in a converted garage is a much simpler appraisal problem and you can probably get away with appraising it like a house w/ feature. But the more complications that are involved the more complicated the analysis gets.
Let's say the subject is (3) units with 2 being residential and the other being a storefront or office or auto service. As a single property, the contributory value for the residential units will not be based on individual SFR or 2-4 sales, but rather the units from a 5+ multi-family property which will also not be eligible for 1-4 financing under residential terms. Same with the auto service or storefront unit; ideally you'll be looking for rental and sale indicators for multi-tenant properties of comparable overall size, not individual single tenant properties.
Mixing the incomes, mixing the different expense ratios and the different cap rates for these components. Unless you actually have enough mixed use sales data to stick to the single set of sales data.
I just got done with a mixed use (6 apt units+ storefront). I had to do 2 separate rent surveys. I had plenty of mixed use sales transactions in that area but I added a couple multi-family of comparable age and unit mix in order to demonstrate the similarities in value for each (which often doesn't happen that way in other areas).
If there are rent controls in place in your local that can REALLY complicate the analysis.
Whatever you do, don't use the salesprice itself as your unit of comparison (which is how SFR and 2-4s are analyzed) unless it actually is an SFR + addition type of situation. That's the mistake I almost always see SFR appraisers do when they appraise different property types that aren't marketed or analyzed that way by the market participants.