Some apps in the Play Store won't update or not even show up when searched if your device is rooted or you're using a custom rom. And if you're not willing to download apk outside Google Play you can use "Aurora Store", get it from f-droid.org. Apks come directly from Play Store, you can even download old versions by inputting an app's version code.
A few days ago I've added B4A-Bridge to HUAWEI AppGallery after requested by one of the developers. It was simple to do. For now there are only a few downloads from there but lets wait and see...
I tried a few and most of them seem to have their own kind of audience but in general it's a lot less installs than via Play.
here some experiences/stats (I don't know exact the best install count on Play but it must be between 45.000 & 50.000. It's hidden now?)
Amazon:
interface and addition of apps is easy with just a small panel with a few tabs and they improve it every few months.
target: global
best download: 541
Opera:
interface is simple but really old fashioned and is like a one page form.
target: global
best download: 854
AppBazaar:
clean and simple to use interface
target: India
best download: 231
Apptoide/Catappult:
This one has potential and it posts to multiple stores.
They have a mechanism that replaces google play billing with their own payments services in your apk.
Sales are in appcoin tho but I believe that you can payout in cash when you have $1000 (I will never make that but the apps might get some ad revenue)
Target: global
Best download: 95
Appland:
I removed my account there probably because it was useless, buggy or not giving me downloads.
SlideMe:
They are really selective and refuse a lot of apps when they think the app is similar to one that is already in their store.
I removed my account there aswell.
I signed up at Huwei but haven't uploaded anything yet. I will do when my apps are updated (I'm busy with it).
Edit:
I forgot to mention the annoyance of forced screenshot sizes which are ofcource different than those for GooglePlay/AppleStore.
so it's a lot of work to maintain all of this.
A few days ago I've added B4A-Bridge to HUAWEI AppGallery after requested by one of the developers. It was simple to do. For now there are only a few downloads from there but lets wait and see...