One of the most damaging pieces of advice I ever received was "starting lots of things and not finishing them is bad".
Rather than finish more things, I became wary of starting things. This did not improve my overall output, and in fact lost me the experience I would have gained from many started-but-unfinished projects. It's taken me years to unlearn this advice in practice.
I especially take issue with calling out practice of a musical instrument. Ten minutes every day or two is much better than one hour every week or two.
I prefer starting as many projects as I possibly can, because it gives me an endless amount of interesting things to work on. Same with writing as well, dumping things on hackernews is just potential draft blog material to write about later
My view is to go all in or not at all. Nobody rewards you for doing a half-assed job. Having many side projects lets you pinpoint one that's particularly interesting and push it to the limit.
I prefer the mantra "One who puts on his armor should not boast like one who takes it off."[0] I take it as advice not to talk about anything that isn't finished, which spurs me to finish more things so I can talk about them.
[0] Disclaimer: From a really old book, might be superseded by newer material (1 Kings 20:11)
To reinforce your point, the act of talking about a work-in-progress can actually give your brain a reward similar to actually finishing a that task, sapping your drive to finish things and increasing your drive to start more things, just so you can talk about them.
Before I picked that up, I started a lot of things and talked a lot about them. Now for any endeavour I find myself making early decisions to push through and finish or move on ASAP. Among other things, it's meant far less time spent on books I don't actually want to be reading.
Rather than finish more things, I became wary of starting things. This did not improve my overall output, and in fact lost me the experience I would have gained from many started-but-unfinished projects. It's taken me years to unlearn this advice in practice.
I especially take issue with calling out practice of a musical instrument. Ten minutes every day or two is much better than one hour every week or two.