Vaillant, Worcester, Viessman etc - who has the best warranty?

I am in the same situation as you (see my post) and have looked at all the above brands you mention. Apparently WB are not fully Opentherm compatible and Vaillant need an aditional component (VR33 i think) to work with opentherm thermostats (Nest, evohome etc). But with VR33 the warranty goes void.
Fully Opentherm compatible brands that I have considered are Ideal, Intergas and Atag. I've read on Atag and they're big in the Netherlands (where opentherm is mandatory) - and in the UK they are well known in the commercial sector but relatively new in domestic market. I have also read good reviews of the Ideal vogue.

I had an installer out, I specifically chose him as he said he fits all the main brands.

So he recommended Logic plus surprisingly. He argued as following, Ideal had a bad reputation with Isar so they came out with Vogue which is an excellent boiler but now gen2 Vogue has poor internal parts - more plastic than brass and copper.

He said Logic plus 10yr warranty jobs done is opentherm compatible.

He said why pay 500 plus more for Viessman or Vaillant, he did mention Wocester but said they are average now.

He used to fit Intergas (which is one of the reasons I called him) but said great product but one flaw the HEX gets scaled up as its always cooling, warming water and after a burst of initial warm water, the water goes lukewarm before heating up again.

Finally he argued I need a filter attached and he said waste of money, they leak.