Bubble was no way to go for me a few months ago (last tried in march), it was a bit ugly, slow, no comparison to Gizmo.
Now (Gizmo don't really works (at the moment?) with MC19) and after finding your post here, I gave Bubble a second try.
First: It works.
Second: There were some steps forward in behaviour and appearance.
Last but not least: Now I'm able to control the volume over the physical buttons (tab or phone), Gizmo was first in doing this so far (for me).
What I really miss is fading in/fading out when I choose the next track. I don't know a program/app (exept Gizmo) with this possibility (don't works with Gizmo and MC19...).
Thanks for the hint, made my day.
I find they look similar (All three are apps with a dark background that list your media with cover art and some play controls) but looks are a matter of taste anyway. As a graphic and web designer there is not much to any of them graphically. But since you mention it the play pause stop next previous buttons themselves in gizmo from a distance look like 5 white squares because there is not enough contrast between the background of the button and icon on them. If you look at how those buttons look on JRemote or Bubble you see they made them Bright on Dark so its easy to see from a distance,A few feet (or when you are a bit drunk) And then there is that black and white two fingers touching icon (sorry to say but it looks terrible especially on high res displays). These are minor issues granted but knowing a thing or 2 about android I ended up modifying those buttons and replaced the icon with the same icon that you see on your windows desktop. This is one fun thing about android when you know your way around
Though honestly I wish they would get around to that some day, I literally did it in 10 minutes. But as my dad once said "If you want something done DO IT YOURSELF!".
As to your other issues. Gizmo works for me with MC 19, Fade between tracks also works with Gizmo (it must be your settings somewhere) I got mine set to 6 sec aggressive crossfade. I don't know why its not working for you. I'd maybe comb through your settings on JRiver and see. Gizmo also got an update and earlier today JRiver also got an update. Maybe those might help.
I use Gizmo everyday to play music at home, It's stripped down interface is perfect for this purpose.
I use Bubble to play video cause I can see the metadata and the folder view is easier to read but really its the metadata on a long press or when tapping the now playing that sold me on it. The other thing (And this is because I am lazy and forgetful) is when I start up Bubble it is where I left it. So if I was watching Breaking Bad S05E11 its right there on the Now Playing window and I can just resume it without having to remember what Episode I was on or what I was watching last. So its like my video bookmark
They both have their strengths. Gizmo is not DLNA so does not require DLNA certified gear. Sometimes that can be a big hindrance with non DLNA complaint hardware (especially the router) that will slow down many DLNA control point apps or appliances to a crawl. So its great they provide a solution that will/should work for everyone no matter what you have for gear. Bubble has some Tweaks but if your Router is DLNA complaint is should be as snappy as Gizmo. I had issues with many DLNA apps before upgrading my router to one that was and that made a world of difference.
One tip for BubbleUpNp disable the thumbnail caching. It should speed things up,Especially if you have a fast network. And if it turns out your router is not DLNA complaint it has another tweak to blast stay alive messages so it keeps the connection between it and JRiver. Checkout the network/upnp tweaks settings in Bubble. If its one thing that app doesn't lack its settings and customizations to make it look and behave as you want.
About gizmo though,I'd maybe uninstall it and reinstall if from the playstore. That at least should be working flawlessly in every way.