Should I trust this video?
It was surprisingly good! Nothing blatantly incorrect there. However, some things were left out or mentioned very briefly, but I don't really blame the video for that - it would've been impossible to describe an entire phonology in depth in less than 10 minutes. The only claim that really made me say what
were when he brushed off the diphthongs because "these are spontaneous observations rather than rules". I don't know what he means by this.
One thing I'm not sure what to think about is the fact that he didn't mention the consonant gradation. On one hand, I can see why he didn't want to go there - it can sound complicated and unnecessarily confusing for his viewers who probably aren't even learning Finnish yet. On the other hand, it's near impossible to explain /ŋ/ or /d/ without consonant gradation, and as a perfectionist, this is not okay.
If you know anything about phonologies in general, it should strike you as odd that Finnish has a phoneme that can only be long but never short, or that we have one voiced but three voiceless stops. I suppose he wanted to keep the video short and simple, but I would've wished he'd given a little bit more time to /d/, especially since it has a cool
* backstory. It's not like he wouldn't have had time for that because he talked about F, G, B, W, and Š being used in loan words, mentioned Ž as "relatively uncommon", said double V, H, and J are possible but rare, demonstrated the different allophones of /h/, and even talked about the syntactic gemination that is almost always left out in videos like these**. I mean, he clearly had time to discuss all of these, so why left out /d/? I don't think it's because he didn't know, because he had marked <v> as [ʋ] and used <x>, /x/, and [x] correctly, so he must know at least the basics of linguistics and so he couldn't have missed it.
* my opinion, but it is cool!
** I might be wrong, but I have a feeling I haven't seen it discussed too often.
All in all, it's a nice video. He has attempted to summarise an entire phonology, which has its consequences, but he has done a good job. He has managed to include lots of interesting and important details, like the syntactic gemination that I was really surprised to see. This is more than enough to familiarise yourself with Finnish phonology.