At 14 con (which is a reasonable to high con for a caster), there’s a 35% chance to fail this check. Even with advantage (e.g. war caster), there’s about a 12% chance.

If your DM consistently sends casters with higher con saves, he just really doesn’t want you to ever break it.

And that’s coming from someone that has a cloud giant wizard with a +10 con saves as the BBEG last session.