Farrow & Ball paint user here, after years of Dulux trade emulsion etc. Like you I heard all this conflicting info and views ... so I decided to throw some ££s at the issue and do a test. I picked an F&B colour and bought a tin of estate emulsion, a tin of modern emulsion and a colour matched tin of Johnstone's (which the rumour mill said makes the best match). My findings were:
*estate emulsion, the F&B classic flat matt finish, 2% sheen. Brilliant
*modern emulsion, 7% sheen - the extra shininess makes it look a slightly different colour as it's all to do with the way the light falls onto the wall and interacts with the paint
*Johnstones - sort of a colour match, but not really, much greyer, plus it was not much cheaper than the real thing, which surprised me
On decorators views. What they say and what they mean are sometimes two different things. When they say 'I don't like F&B' what they mean is 'I am worried the client will complain when they get a scuff after two weeks and it won't wash off ' plus 'I can't get a trade discount on F&B so my margin is affected'. I just did a big decorating job - and the decorator I hired was not the cheapest, but the guy who arrived with the F&B colour chart in hand and said 'Colour X, Colour Y, these are brilliant choices and it will look great'. He did a great job too!
On touching up. Again, this is to do with how the light falls on your wall. In some lights I can faintly see that there was once a door that has been plastered over underneath one of my walls. This is not to do with crap plastering - it's to do with light. Most of the time this effect is not there. In another room there is a dirty fingermark in the middle of the wall, so I touched this up out of a tester pot. Again, in some lights you can see it, some lights you can't, mostly not.
On F&B vs Little Greene vs Mylands vs P&P Library - all of these more expensive paints have great pigment in them and are in more trendy colours and finishes than cheaper paint. You are either the type of person who's willing to shell out on the expensive stuff for a better look (might be the finish, might be the colour) or for wealth and status-type reasons (meaning you can show off your F&B paintwork to the neighbours), or you're not. IMO, with all these paint brands it's less about which one is technically best and more about 'am I the sort of person who paints my walls with these types and brands of paint'.
On undercoat. Don't use white undercoat for one of the darker F&B colours, but white undercoat will be fine for the whites .. as long as it is a water-based type of paint.
On marketing: F&B's marketing is beyond good. They set the trends with their social media, paint consultancies and so on. But that doesn't mean their paint isn't any good.
I think your decision is not whether to try F&B, but whether to try the estate emulsion or modern. Sorry, that probably confuses it even further 