It is effectively disadvantage to the roll. Hex invocation doesn't give a save and has similar effect.
The big difference between disadvantage and -5 on the roll is -5 on the roll is not countered by advantage. Also, Hex, a 1st level spell, only gives disadvantage on ability checks for one of 6 abilities to ONLY one target. Not disadvantage on all attack rolls and saves to ALL creatures within 30 feet. And its version of disadvantage is not countered by advantage. That is a very powerful effect.
Slow is a 3rd level spell that halves moment. This Frozen effect cuts all movement to 5 ft per turn. That's a higher level effect.
I'm just curious why this stuff is so non-standard. I assume it's balanced by something else we haven't seen in the creatures. It's just unusual.