I would love to hear other people's journeys in finding that grail? bass.
My "alpha" experience with boutique builds was with a custom order of a non-boutique. Back in my 80s metal days I played a Jackson Concert Bass in a music store. I loved the bass but not the color, so I custom-ordered one. I was nervous but it turned out fine, which removed much of my apprehension toward custom orders (and that eventually included online ordering, either new or used).
My first true boutique order was the Sadowsky NYC P5 in my avatar. I had played a friend's Sadowsky 4 string and loved it. The 5 string specs were to my preference so I took a chance on the P5, and that bass is still tops for me. I then purchased two more Sadowsky NYCs, also great.
My first boutique order where I had no first-person experience with the model in question was with Darrin Huff.... yes, that guy. Over 20 years later I got a 33% refund of my down payment... I offer no opinion on whether I think I'll get the rest.
The Huff experience has been unpleasant to say the least, but it didn't deterred me from buying from other smaller boutique shops -- however I made sure to order from builders who had a well-established track record. These include Nordstrand, Zon, Martin Keith (twice). I've been unconditionally thrilled with all basses, and build times for all were reasonable: 1 year or less.
Two sister basses bought from a different small boutique shop were... let's say four stars out of five because I had to have a tech mod them to correct some minor issues. I won't name the builder because these custom orders were far less expensive than those mentioned above, both arrived within 6 months, and after the corrections the basses are what I hoped they would be.
To sum up: with one notable exception, my experiences have been very good to superb.
Context: I know the specs I want in a bass, and only order from those who can provide it. For me, it's five string, 1-7/8" nut width, 19-20mm bridge spacing, moderate to chunky neck profile. For balance I need an upper horn (or single cut) that comes close to the 12th fret. Weight: 9 pounds, give or take a bit. Obviously these are MY specs; point is, it can be helpful to know your own preferences... unless you're lucky enough not to have any!