no cons after 4 years of slime (suzuki 250 across) and now the 800gs (first 800gs batch in Australia). No puncturs as yet, including several 1000km on dirt tracks... at least.
I have repaired a nail through a car tubeless tyre, but not a motorbike.
I wish they were tubeless on the 800gs. BUT there are ways to convert to tubeless...with reservations (some market developed products).
Short and straight, never seen evidence of SLIME'S use/presence (sealing a Tyre due to slime).