I've used Delta both with a gamepad (Backbone) and just with the touch screen to play a few Gameboy Advance games. I have an iPhone 12 Pro Max. The experience was super nice both ways.
The touch screen interface is essentially the shape of the gameboy or whatever Nintendo console you're emulating and is responsive and works the way you expect it to. The Backbone connects automatically and just works. If you want, you can remap the buttons.
The annoying thing about having to use Delta and Altstore is the need to keep AltServer running on my desktop or laptop. Frequently, my phone would be unable to find the running instance the only way to refresh the app (required every 7 days) would be to connect with a wire to my desktop or laptop. When it worked, it was great. But when it didn't, I was annoyed.
That said, it does have cloud sync with various services and it does work well.
While travelling, being able to have a rather large library of classic games to play, while using the device that I'm already carrying is a huge benefit. While I wish emulation on iOS was a little easier, using Delta/AltStore is not difficult or anything.
I've carried a Backbone with me and played a ton of GBA games with touchscreen controls and the backbone, and it's pretty great, depending on the game. Carrying the backbone is a lot smaller than a Nintendo Switch or one of the many larger screened android emulation devices that are now available.
This is all up to personal preference obviously, but having one less device to carry is a big plus for me. Game emulation is the one thing that makes me question whether or not it's time to ditch iOS and just get an Android for my daily driver...
I recently replayed most of Pokemon Fire Red in RetroArch on my android phone (installed via f-droid). It worked surprisingly well! I only stopped because I bought a dedicated device to do so (Anbernic RG35XX, which is also fantastic). So in a pinch, console emulation on a smartphone, especially for older consoles like Gameboy Advance, works very well.
To get around the on-screen buttons, I ended up connecting my ps5 controller via Bluetooth which worked amazingly straight out of the box with zero configuration.
Anything portable has always emulated well (or at least have since 2012).
Virtual buttons exist for the gamepads of older devices (NES-derived layouts generally look pretty good, so NES/SNES/GB/GBC/GBA emulates well).
The 3D home consoles tend to be a bit more dependent on the game (early 3D games have some pretty shitty button mappings in general and that's only amplified when emulating with virtual buttons).
As for emulation quality; handhelds emulate really well. Consoles made after 2000 are a bit of a toss-up at times, but generally also work decent.
As for battery drain; not an issue with handheld/old system emulation. Can't speak to home console emulation sorry.
I can play Wind Waker on my Pixel 4a, a mid level phone from a couple years ago. I imagine if apple could be arsed to open their stupid ban on jits you could emulate switch titles without a hitch on iPhones.
You probably still want a game pad for any 3d titles tho, on Gameboy on screen inputs are fine but the lack of precision can be really annoying to get over. Fortunately the xbox controllers work out of the box with dolphin.
I see, the last time I looked into this was shortly after apple closed the last known jailbreak options and before they allowed jits in some circumstances.
The lack of a speed difference I assume is down to the fact that delta supports some pretty weak consoles which you could emulate in a browser. Dolphin's jit vs interpreter vs cached interpreter in for example Wind Waker is a 10-40x speed improvement on my phone.
