Personally, I don't think nematodes are very 'nice'.....I don't think they kill the slugs in a very 'nice' way.
But then, I'm really anti-killing-anything in the garden! I've just forced myself to spray the hideous black infestation of aphids on my poor Kentish bell flowers (which are being harvested by the ants running up and down - apparently they actively 'preserve' aphids for their honeydew over winter - honestly!), but didn't like to do it. That said, I can't see any difference at all anyway. I'll just have to cut the blooms off and bury them in the green waste for the tip.
I do hate 'infestations' of anything - something really creepy about 'en masse' insects etc.
I don't know wether this would divert slugs from flowers, or just encourage more of them to breed, but I find that if I leave any old cat food out (for the magpies - boy, do they love old cat food!), the slugs come from all corners to join in the feast.
(Off the top of my head I doubt the nematodes would adversely affect whatever eats slugs - v. brave, whoever eats them! - on the grounds that the particular slug-nematodes may not be parasitical on other animals.....because if so, why would it be legal to sell nematodes against slugs, if there were a risk that we could be infected as well?? But that may be spurious reasoning)
(Nematodes, by the way, are seriously to be avoided as a human parasite - they are really 'nasty' - think tapeworms (but nematodes are different biologically). They are VERY infectious. They like raw fish, so always check fish for them - sushi can be a source of infection alas. I once bought a piece of white fish, had it laid out on a plate while I prepped the veggies, then when I went back to get the fish I saw a pale, white, very small but clearly visible worm wriggling on the surface. I was seriously shocked and horrified. Binned the fish.)