Euls is a decent item only if you know the other team has bad cooperation and timing. These are usually the opponent's own disablers such as Lion or VS. The only time I can think of Guiensoo being much better than Euls is when your team already has the advantage in numbers and want to pummel down your opponent. Also, that extra second can help you chain your team's disables really well.
Don't you mean Euls being better than Guinsoo?
Anyway, Euls is a really good item when chained with Kelens. It can also help in terms of survivality when being ganked and stalling until your allies can come and help you.
Altho I still think Guinsoo is generally better for various reasons.