Fantastically straight forward? I strongly disagree. I've done the SA many times, and trying to find out your balance or payments list or anything useful is now a kafka-esque nightmare. Last time I knew they must owe me money but they didn't say at the end, and it took me about 15 minutes to find out how much, with no indication of when it would arrive. At the end of the SA they even had some sort of message saying make sure you pay what you owe, which turned out to be nothing. This is all complicated because you pay on account, paying the next year's tax 15 months before you submit your return (they guess your next tax bill based on the previous one, as you don't submit the return for 9 months after the end of the tax year, so you don't get in too much credit).
The new SA process is no different from the old one, with minor cosmetic enhancements, but they've made it almost impossible to get to the old account sections.
Notice how there's no login button. On a website that relies on logging in. Now search for "self assessment login" or "self assessment account" or "self-assessment balance".
See any login button? See anything resembling a login? last year you could still get to the old account screen after some hunting, but that's gone now. The new account screen is a mass of text with links sprinkled through out, where the old one was a compact menu.
Plus, when you finally do find your accounts, you can only see one year at a time, have to use a drop down to change year, the amounts you owed are on one page, they amounts you paid are on another, some of these figures are still provisional, and nothing adds up.
As for submitting your company accounts, what a joke that used to be when they re-did it. I knew exactly what type of account I needed to submit. But to get to that form, I think they had a labyrinth of 16 web pages, each just one question per page, that you had to answer while they unhelpfully tried to pick the form you needed for you. Get the wrong form? No way to go back to see what answer you got wrong, no way to fix it, you just had to start again. It meant I had to go through it 3 or 4 times before I finally got the right form.
Before they "renovated" it, you just picked the form you needed.
They even did a whole blog post about how great the one page per question concept was, which turned out to be utterly bullshit.
I've lost all faith in the gov.uk team, I feel they're trying too hard to be cool and "innovative". They have thrown out convention, and are often making poor UIs and user journeys because of it.