Geoff Lindsey has some damn great videos on English phonetics and phonology, I heavily recommend them; for example, this one, about the transcription of Standard Southern British vowels; to keep it short

1. the diphthongs should be treated as vowel+consonant sequences, not as one vowel transitioning into another.
2. /i: u:/ behave more like diphthongs than long vowels, and should be transcribed as such.

On the video shared by the OP, I think Lindsey made a great job highlighting that prosody is also a feature that a language may or may not use to convey information. English is mostly stress-timed (unless you're Welsh, and specially if you also speak Welsh), so it uses prosodic stress a lot; in the meantime, Spanish (he compared EN and ES in his James Bond example) is mostly syllable-timed, so it barely uses this sort of reduction - if anything, quicker speech tends to attack consonants (cue to Chilean [äo] for /aðo/ -ado) way more often.

He also mentions L2 speakers; it's worth noting prosody is a fucking pain to assimilate, so it's no wonder that non-native speakers rarely use it. (Also, anecdotally speaking, I've noticed people highly proficient in a non-native language tend to let that language affect their native one's prosody.)