Maybe they'll pick up L*bour voters who like to swing green in Engl*nd but this underestimates the stranglehold that L*bour as on Welsh politics. Only Plaid was able to realistically threaten their monopoly on power at the height of L*bour's election failures.
Green lost a lot of good faith with Welsh members when they refused to split the Engl*nd-Wales parties. There's a reason why they have a stereotype of being Bristolian hippies.