Nintendo have put out an official message on the Wii U & 3DS eShops winding down:
END UPDATENintendo wrote:As of late March 2023, it will no longer be possible to make purchases in Nintendo eShop for the Wii U system and the Nintendo 3DS family of systems. It will also no longer be possible to download free content, including game demos. Furthermore, as this date draws closer, related services will cease to function:
As of May 23, 2022, it will no longer be possible to use a credit card to add funds to an account in Nintendo eShop on Wii U or the Nintendo 3DS family of systems.
As of August 29, 2022, it will no longer be possible to use a Nintendo eShop Card to add funds to an account in Nintendo eShop on Wii U or the Nintendo 3DS family of systems. However, it will still be possible to redeem download codes until late March 2023.
Users who link their Nintendo Network ID wallet (used with Wii U and the Nintendo 3DS family of systems) with their Nintendo Account wallet (used with the Nintendo Switch family of systems) can use the shared balance to purchase content on any of these systems until late March 2023. After that, the balance can only be used to purchase content for the Nintendo Switch family of systems.
Even after late March 2023, and for the foreseeable future, it will still be possible to redownload games and DLC, receive software updates and enjoy online play on Wii U and the Nintendo 3DS family of systems.
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Having owned both a Switch and Switch Lite for a couple of years now, I have to say - in all honesty I prefer the Wii U as a couch/bed experience. The Gamepad is WAAAYYYYYYY more ergonomic and comfortable to hold than either Switch, has a larger screen (no OLED though), and not only has an actual dpad, it's right in the middle like a proper gentleman's console.
The main system didn't get too many shooters during its lifetime, and I believe most of them are available on other platforms as well.
I know these titles are available:
Aperion Cyberstorm
Ghost Blade HD
Nano Assault Neo
Scram Kitty and his Buddy on Rails (This is kind of cool)
SkyForce Anniversary
Teslapunk
Of the above, I'd say definitely give Shinen's Nano Assault Neo a look. (Or the PS4 port, Nano Assault Neo X). It's a twin stick shooter with a Metroid-like sci fi bio/bacteria theme with CRAZY good visuals, and plays really well. I had it in my top 25 last year, people looking for an alternative to pixel art indies/retro should check it out.
Satazius Next & Armed 7 DX were released as the Wii U Shmup Collection in PAL territories, I don't know if they also got a digital release there but they don't appear to be on the North American eShop.
The real meat is in the Wii U's Virtual Console. It never got as wide a selection as the Wii's, but on the plus side it still exists. One caveat - emulation is not perfect. Most titles play just fine, including the shmups, but I did find a couple of others like Zelda II & NES Punch Out!! too laggy for even my lax standards.
But still, there are titles here like Pocky & Rocky with Becky which can't be found anywhere else, and I have no idea if/when the TG-16 stuff will ever surface again either as it's all owned by Konami now.
NES:
Galaga
Gradius
Life Force
S.C.A.T.
Sky Kid
Stinger (Moero Twinbee)
Xevious
SNES:
Axelay
Super E.D.F.: Earth Defence Force
Wild Guns
N64:
Sin and Punishment
Star Fox 64
GameBoy Advance:
Pocky & Rocky with Becky
TurboGrafx-16/ + PC Engine:
Air Zonk
Blazing Lazers
Lords of Thunder
Gradius
Salamander
Super Star Soldier
Final Soldier
Soldier Blade
Wii:
Sin & Punishment 2
Sadly, Irem had some PCE titles there (R-Type, Image Fight 1&2, Mr. Heli) which were delisted a couple of years back.
There are also a few other titles in other genres on the eShop worth getting before they're gone, like the Capcom D&D Collection, or Pushmo World which is a forgotten Nintendo digital-only title exclusive to the platform that may well never see release again anywhere.
There are some other cool oddities on the Virtual Console like Car Battler Joe, Onimusha Tactics, the GBA Klonoa games and Mighty Final Fight. Not to mention DS titles like Fire Emblem Shadow Dragon or Zelda Phantom Hourglass/Spirit Tracks which may never see official physical dual screen hardware which can properly display them again.