When comparing QuickTime vs IINA, the Slant community recommends IINA for most people. In the question“What are the best video players for macOS?” IINA is ranked 1st while QuickTime is ranked 5th. The most important reason people chose IINA is:
It's like mpv, just more user friendly GUI and more settings available via UI.
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Pros

Pro Recording and editing is possible
Unlike other video players, QuickTime offers recording and editing opportunity. To unlock this features QuickTime Pro 7 is required, which is a fee-based software (29.99$). Program features include possibility to record video and audio directly from microphone, FireWire camcorder and iSight camera. Speaking of video editing, cutting, deleting, resizing, flipping, or rotating a movie is possible.

Pro Easy to use streaming features
QuickTime video player has "Zero-configuration streaming" - video player automatically determines best connection speed. Plus, it can automatically reconnect, if the connection was lost. Step by step video file stream or broadcast guide can be found in the source.
Pro Screen recording is a easy

Pro Elegant look and comfortable interface
QuickTime is developed by Apple, which is known for its plain, but gorgeous and user-friendly design. QuickTime is included in the list of default programs on macOS systems, so it is a good free option for beginners and users who love style and comfort.
Pro Posh smell
Look up Posh and see if you think a software movie player should be POSH. What a hoot.
Pro Like MPV with better GUI and settings
It's like mpv, just more user friendly GUI and more settings available via UI.
Pro Extremely stable
Extremely stable, even while still in the beta.
Pro Support for lots of codecs
Supports lots of video codecs, like mp4, mkv, etc, and lots of audio codecs including opus.
Pro Doesn't black out other displays when going full screen
It can black out other screens if you want, but out of the box it leaves your other displays alone.
Pro Runs on MPV so it's scriptable and runs any MPV script.
More information here.
Tip: if you're doing a lot of cropping and slicing of videos and you thought the new quick tools in Finder / Quicktime Player are a great improvement, try setting up IINA with slicing.lua and some keybindings for navigating your video + setting the slicing positions.
Make sure your ffmpeg command (in slicing.lua) is set to copy the original codecs and that it's using your GPU.
Pro Online subtitle searching and intelligent local subtitle matching
Pro Fully customizable keyboard, mouse, trackpad, and gesture controls
Pro Open source
Pro Powered by FFmpeg
Cons

Con Video player can interfere with other software
Leaving QuickTime's video player running in background doesn't play well with certain software, especially games and full-screen software.

Con Some common video file formats are not supported
QuickTime does not have native support for some common formats such as DivX, .wmv, and .mkvs.
Con Can't play .srt subtitle files

Con Crashes after OS X update
Sometimes after an OS X update, if the video player wasn’t updated for a long time, QuickTime can crash for no reason
Con Unstable
May crash unexpectedly, while clicking on something in the subtitles section in the right-hand panel, for example.
Con Blacks out other displays when going full screen
