The rerolling is powerful. More powerful than d10+2. If you roll a one or two on a d10+2 you get a three or four for damage. Wheras if you reroll a one or two with a brutal weapon you get a chance at higher than 3 or 4.
Actually, it isn't. I'll illustrate using a hypothetical d10 brutal 2 weapon to show that it is equivalent to a d8+2 because it makes the numbers nicer, but the same principle applies to show that a d12 brutal 2 weapon is the same as a d10+2 weapon.
The distribution of possible results after a single roll of a d10 brutal 2 is as follows:
20% chance of reroll.
10% chance of 3.
10% chance of 4.
10% chance of 5.
10% chance of 6.
10% chance of 7.
10% chance of 8
10% chance of 9.
10% chance of 10.
If you do get a reroll (20% chance), the distribution of possible results breaks down as follows:
4% chance of a second reroll.
2% chance of doing one reroll and getting a 3.
2% chance of doing one reroll and getting a 4.
2% chance of doing one reroll and getting a 5.
2% chance of doing one reroll and getting a 6.
2% chance of doing one reroll and getting a 7.
2% chance of doing one reroll and getting a 8
2% chance of doing one reroll and getting a 9.
2% chance of doing one reroll and getting a 10.
If you actually do get a second reroll (4% chance) the distribution of possible results again breaks down as follows:
0.8% chance of a third reroll.
0.4% chance of doing two rerolls and getting a 3.
0.4% chance of doing two rerolls and getting a 4.
0.4% chance of doing two rerolls and getting a 5.
0.4% chance of doing two rerolls and getting a 6.
0.4% chance of doing two rerolls and getting a 7.
0.4% chance of doing two rerolls and getting a 8
0.4% chance of doing two rerolls and getting a 9.
0.4% chance of doing two rerolls and getting a 10.
By now, it should be fairly clear that regardless of the number of rerolls, the chance of getting any valid result (i.e. from 3 to 10) is equal. This is essentially d8+2.