With all the free space on the card, it would be nice to just use Momentum's wording instead of saying "have momentum". You could probably put the ability name on there if you wanted too, but totally optional.
Is the Saboteur itself burrowed? Thematically it would fit and make it benefit from its own buff, but that's up to you.
This is actually a really nice use for burrowing creatures. Thinking about whether it's worth gaining momentum but halving you damage is a nice bit of strategy. It doesn't make a mass momentum OP either, seeing as the damage is low anyway. Maybe it's a little too niche but it's still a really nice card.