If you take a look at the encounter building chapter in DMG, you'll see that the difficulty increase isn't linear when you increase the number of monsters. i.e. if you double the number of monsters the encounter becomes more than twice as hard.
As long as you are aware of this it shouldn't be a problem.
A year ago I was DMing Curse of Stradh.
One of the characters was a revised ranger from unearthed aracana.
When the party became level 3 he used his Primeval Awareness ability to find all undead within a 5 mile radius
I used some old school house rules from earlier Ravenloft-versions and decided that not only did he know where all undeads were - but all undeads within a 5 mile radius knew where he was - and they were coming.
So the party of level 3 characters took stand in the burgomasters house aginst all zombies in the whole village of Barovia. I don't remember how many undeads I throwed at them, but it was certainly in the vicinity of 88.
It was quite epic.
As long as you are aware of this it shouldn't be a problem.
A year ago I was DMing Curse of Stradh.
One of the characters was a revised ranger from unearthed aracana.
When the party became level 3 he used his Primeval Awareness ability to find all undead within a 5 mile radius
I used some old school house rules from earlier Ravenloft-versions and decided that not only did he know where all undeads were - but all undeads within a 5 mile radius knew where he was - and they were coming.
So the party of level 3 characters took stand in the burgomasters house aginst all zombies in the whole village of Barovia. I don't remember how many undeads I throwed at them, but it was certainly in the vicinity of 88.
It was quite epic.