I do not quite fit your criteria, a few years too young, although I don't feel it! I do have a pre-teen and an early teen though. But I will try to answer anyway.
Downsides I was worried about were mainly financial. I took several years out to be a SAHP to help exH build his career faster, I would be playing catch up, but with no support at home for sick children, or dentist appointments etc. Would I be able to build up my pension enough? If I had been older at the time, I think I would have felt this pressure even more.
I was worried about being able to afford a decent enough house for us. Choosing something too small, but no work, or something with space, but having to do ALL the diy again! Or find the money for tradesmen etc. Would DC want to bring friends over? Would they end up rather spending time at their Dad's because I wouldn't be able to afford the latest devices, and he buys takeaways all the time?
All of the above actually turned out to be irrelevant. I'm much better off financially, and work lets me shift some of stuff around, so when DCs are with their Dad I can get more done, so need to do a little less when they're home.
The actual downsides have mostly been specific to my situation, exMIL and SIL behaving very poorly, exH continuing his behaviour. It has caused some difficulties in co-parenting, and both schools had to be made aware.
Smaller downsides, the sheer amount of paperwork, and effort it took to get the flipping thing done! As we ended up having to go through the solicitors a lot, it was quite costly too, but I was better off for it as it got me a fair deal.
None of the practical downsides mentioned above really applied to me. I was already doing all the housework and cooking, and most of the childcare, whilst working full time. I never had support when ill, and I had to do all the diy myself. After divorce I just had no one picking holes in everything, or giving me extra chores. Cleaned the kitchen? It stays clean! I also slept much better. I could write an essay on the upsides for me, but I won't. But they greatly outweigh the downsides.