It's not immunity if the developer does not want it to be. +10 is clearly too much, in retrospect - I was trying to find something that scaled well but started at a significant amount of saves to be worth casting, but overdid t - but a bonus to saves does not act the same as a second dice.
Here's the formula for save success with two d20 dices:
([chance to succeed roll]+[chance to succeed roll]*[chance to fail roll])*100%=[chance to make save]
If we input values in it, you'll notice it pushes everything up to the top very quickly. Even a monster with a success rate as low as 50% reaches a success rate of 80% before accounting for SR. That's without adding the +5 to saves you've put in there (which raise that to a 88% success rate). That's all fine if you want low success rate, but that's not what is wanted. If it's too low, it's just annoying spam until it lands.
And, that's the problem. For as long as the saves are well set, my incarnation would still lead to decent success rates. Yours, however, would quickly push about everything in the 70%+ range. Mine, if we lower the bonus under +10, wouldn't.
Some monsters could be using a low-level version of it (clicky, pot, whatever), which would ablate faster...maybe in only one or two castings.