A lot of meta threats (Mienfoo, Pawniard, Grookey, etc) exert a lot of pressure with their super effective moves that Lileep can't handle. It's Knock Off bait (i.e. crippled without its Eviolite) and Knock Off Pawniard is one of its safest switches: Eviolite Pawniard isn't too fearful of a non-STAB Earth Power (plus it's immune to Toxic and resists Giga Drain). Any priority after a Knock Off pretty much kills it (Pawniard and Grookey say hello again), so there goes your Lileep.
It's actually pretty good if you need a Water resist, being able to sponge up Mareanie and Staryu Scald. However, Dewpider and Frillish are arguably better. Frillish gets helpful utility moves like Scald and Will-O-Wisp, while Dewpider gets the amazing Sticky Web for a dedicated lead set.
In short, Lileep struggles to keep up in the offense-dominated metagame and is outclassed by other Water-immune Pokemon.
Hope I helped!