I started to see a lot more complaints for it around the time of Noctis(First super powered character), in which case, he was our 7th in the pool(Excluding limited edition units). And as you stated, the pool is simply too small.
But the issue is the purely useless units from it. Hence why I always thought the best compromise was altering it till the unit pool expanded.
Rather than using the base, they use the max growth. Yellow crystals will only produce characters that can level to 5*, and Rainbow 6. Blue would be unaltered. This would mean Yellow crystals would see the drop of the likes of Vive, Penelo, etc. And Rainbow would see the drop of any unit that caps at 5*.
So while a Rainbow may still be Chizzy or ExDeath. One is that it will at least be a unit of usefulness, two is that it will drastically increase the odds at a base 5 unit. While not only offering one of 10 units.
Then in a few months, once they get to TT. Switch it how it is in JPN.
The other thing I would like to see is this, for Limited Timed units, I think offering a boost to drop would be a nice idea over the typical banner. It would incite more to pull and hopefully maybe give people more reason to pull, than hoard for the best in JPN. What I mean by this is, for Limited time banners, the base 4* has a 10-12% drop rather than the usual 6-7%, and the Rainbow unit will have 2-2.5% drop, rather than 1%.