Pros: you get exactly what (you think) you want
I was going to say this:
Pro: You get exactly what you think you want.
Con: You get exactly what you think you want.
Also, if you get an already-made bass, you can play it before you buy it.
If you get a custom, there's always the chance you get it and it's not as amazing as your wallet says it should be, and by then it's too late. And that isn't necessarily because the build is bad. It's just every bass is different and it could be built perfectly to your specs and then when it's in your hands and ears it just doesn't gel with you.
I've had 5 custom basses made for me. LOVED three, meh on one, and kinda hated the other. That "hated" one was a Carvin 6 string in the 1990s that i saved up a bunch of money for and then ordered how i
thought i wanted it, and then compromised a bit for other things due to budget reasons, and i loved that bass because it was my 6 string, and it was my money, and it was my custom. And i hated it because, i kinda hated how it played and how it sounded. And for decades it was one of my two basses. I had no way of playing a Carvin before i ordered. It was what it was, i wanted a 6 string because Les Claypool, and in the 90s, obsessively re-reading their catalog must have had an effect on me.Turns out, when you think you know exactly what you want when you're 16, you don't.
With my Olintos i knew pretty much what i was getting even though it was custom because i played a bunch of their basses in the shop before i ordered mine, and talked a whole bunch with Eric and Jimmy before ordering about what i like and don't like and why.
My Maruszczyk Frog fretless i got sight-unseen and when it arrived it was everything i hoped for. But spent a lot of time thinking about what i want from a fretless, and learned a lot from buying the Carvin. I had a clear vision for it and every decision played into that vision.
The "Meh" custom was sight-unseen, and it was kinda my own fault because i didn't have a clear vision. It was also a Maruszczyk and it was built properly and all. I just wanted a bass and had the money, and it arrived and it was a fine, well made bass that i just didn't love. I was experimenting with the options i picked. It was a model i hadn't played before. Somebody else might have loved it but it just didn't speak to me.