I will start by saying that
everyone has had different experiences,
As much as I rearrange the hard-scape and do water changes, I'd think my tanks would be a MESS with soil in them.
but in my soil-capped-with-gravel (or sand) experience, as long as you 'plant it and forget it' and don't rearrange things much, it works out fine. Water changes are no big-deal, but you can't do a normal 'gravel vacuum' like you can without soil. The mulm buildup is no big deal and helps the plants, as long as its not excessive.
That's got to wreak havoc on your impellers and filtration too. Dirt is mostly ultra-fine rocks/sand (super abrasive).
I've only used sponge filters driven by submerged powerheads in my soil-based aquariums. HOB type filters can work (just not worth the hassle of the CO2 they offgas, so I don't use them), and as long as you don't stir things up, should not be a problem for impellers. I've never had an impeller fail on one of the submerged powerheads. When I do water changes, I take them out and rinse them in the change water, using a Q-Tip to get into all the small places between the blades.
I also don't get how the two separate substrates will stay put.
As long as you don't do aggressive gravel vacuums, or have fish that dig, they stay seperated just fine.
I don't think soil is worth the hassle, personally. I've kept planted tanks for 20+ years (before that, marine tanks). I've kept 3 soil-capped-with-gravel and 1 capped with sand, from between 2 and 6 years. Sand was easier to me, between the two. But my preference, by far, is simple gravel for plants. I like to rearrange things, grow fast-growing stem plants (that I give away or trade at the LFS), etc. When you pull up a crypt that, just a few wks later, has put out 5 miles (ok, that is an exageration!) of roots in the soil, its a mess trying to pull it up to give/trade.