They do seem to have made a big mess at Euston and I'd say the screens are unnecessarily bright too.
They are very hard to look at because they appear to use a technology intended for viewing at a medium distance (e.g. up where the old board was) for viewing up close. For close viewing, TFT LCD or OLED are better technologies (probably the former as it's cheaper and doesn't suffer screen burn-in like the latter does). They are indeed too bright, but also too low resolution for up-close.
Brightness is probably adjustable in software, that said.
Someone really screwed up here and spent a fortune on something that works very badly.
Paddington has had side screens and no main board for years now. It's not half as convenient imo.
It works OKish at Paddington because the boards aren't placed such that they result in everyone standing in the way of the main flows, though. They also use a technology (orange LED) that works for close viewing, and they have enough panels.
Paddington is also a bit different from Euston in that it has a very long concourse - imagine if Euston's concourse ran all the way from P18 to P1 rather than just being the middle bit? With that layout, distributing the displays can make more sense. Waterloo is even more extreme in this regard, which means it has several different boards, some low level and some high level, because there's not one location where you can put one to be seen from anywhere. Euston is different - if one is placed where originally intended, you can see it from everywhere other than inside shops and in the tunnel.