I can only speak from my own experience, which didn't seem like much until I did some math and realized I've been writing online for like 6 years now. Gee, that went by fast!
My first online novel, "No Man an Island" was almost entirely complete when I started posting it. I highly recommend this for complete stories (as opposed to ongoing serials) because then you have a structure and can easily meet your posting schedule and your audience's expectations. In a serial I would adapt that advice and have Book 1 or Arc 1 finished and then work on Book 2 as you post 1. It buys you time for editing, plot arcs, and life events.
My audience's feedback led me to create an additional 100 pages of material to flesh out the story, and they were very forgiving of the fact that I revised the beginning a half dozen times and included bonus chapters in the first half. They didn't "know" that they helped me revise the second half before they ever saw it, but the whole time I learned from our interaction how to become a better writer, for which I am grateful. That's the other advantage of writing ahead, it lets you adjust along the way without disrupting the schedule.
Now, as soon as NMAI was done I started a "serial" instead of a novel -- "The Surprising Life and Death of Diggory Franklin." I wrote it spontaneously, on the fly, and it grew and grew and grew. I find it a much more organic process, as there wasn't the pre-planning and editing that I did with NMAI. However, I plan and think and brainstorm about things all the time, it's just during the series instead of before.
The biggest problem is that I have no buffer in case of life crises -- and this year I had three, which slowed my writing to a halt. That dropped my audience to almost nothing. If you want readers, do your best to KEEP your SCHEDULE.
Luckily, I've had readers this week now that I've had regular posts the last three weeks, so sometimes audiences might be forgiving. But it's probably better for your relationship with readers to not need their forgiveness.
I had ZERO plan with Diggory and actually intended it to be a short series, maybe one or two books. It's at something like 14 now, so that didn't work out - serials on the spontaneous fly tend to grow and grow. So there's a lot of creativity in the process -- I would compare the pre-planned novel to a structured architectural work (only NMAI was experimental, so something like Frank Lloyd Wright) whereas the serial expands like a fractal -- taking on new complicated shapes from the original matrix over time.
You risk painting yourself into corners or creating plot-holes that way, but I compare new chapters with old ones that are interlinked on a constant basis so I don't lose my way. A complete arc has the advantage of knowing already where you're going, so you have a map and a compass. On the fly means you take detours and have to reorient based on the landscape, since you're making the map as you go instead of knowing it before you take the journey.
So it depends a lot on personality -- I find they're both very different stories for me, and taught me very different things. Both were worth it, for entirely different reasons.