OK, had a look at the data and tactical2017's suggestions were "correct" (their suggestion was either won or came 2nd) in 573/632 constituencies (leaving NI out of it).
They were "wrong" (i.e. their suggestion came 3rd or worse) in 59. 58 times it was suggesting LDem, the only other one was the suggestion of voting Green in Isle of Wight (3rd behind Con and Lab).
Also, of those 58 of them it didn't matter as the Conservative winner got more votes than the 2nd place candidate and the candidate tactical2017 were suggesting.
The one remaining constituency was Portsmouth South were tactical2017 suggested voting LDem but they came third, but Labour still won (with Tories coming 2nd).
tl;dr tactical2017's suggestions did not split the vote and hand the Tories a win in any constituency
That's good. I was torn in my constituency where Con were holding, and Lib was the suggestion on the tactical voting machines. A friend of mine who is involved in local politics said a Lab swing was more likely so I voted Lab. Con won, then Lab, then Lib.
OK, had a look at the data and tactical2017's suggestions were "correct" (their suggestion was either won or came 2nd) in 573/632 constituencies (leaving NI out of it).
They were "wrong" (i.e. their suggestion came 3rd or worse) in 59. 58 times it was suggesting LDem, the only other one was the suggestion of voting Green in Isle of Wight (3rd behind Con and Lab).
Also, of those 58 of them it didn't matter as the Conservative winner got more votes than the 2nd place candidate and the candidate tactical2017 were suggesting.
The one remaining constituency was Portsmouth South were tactical2017 suggested voting LDem but they came third, but Labour still won (with Tories coming 2nd).
tl;dr tactical2017's suggestions did not split the vote and hand the Tories a win in any constituency
[EDIT] FYI @Velocio