If you consider pioneering to mean 'becoming popular' then sure. But they didn't pioneer anything, they're just the ones who caught a break.
Comparing Slayer and Metallica and Venom is like comparing three different important bands in their own right. Venom are the pioneers. What Slayer did was set up a band with a guitarist who has solos that continue sound pretty much the same as they did on Show No Mercy, and have bands come in and try to compete with Kerry King as being the lofty solo guitarist. Metallica had a better solos in some respect, but they were of a different sound and they were the start to another part of metal. The competitive aspect of it is evident why Metallica and Slayer have been in competition since their bands first started, and that can be heard throughout their discographies. So neither one of them were 'pioneers' in that respect. The reason I say that Nocturnus is better than Slayer is because they did what Slayer was trying to do, better than they did if by anything, by Nocturnus' lead guitarist simply being better than Kerry King's solos. Nocturnus made a statement with their two albums as a response to Slayer. Kerry King isn't that great of a guitarist. So really, the only reason Slayer is an influence is due to their popularity and not due to their musicianship.
Why are you only talking about the fucking solos?