Honestly, we can't really gauge how strong it is because we don't exactly know how much energy these fighters contribute to the spirit bomb. In the sense, none of the fighters were tired or out of energy after contributing to the Spirit Bomb. Plus, it was essentially missing the energy of Universe 7's 2 strongest fighters "Goku and Vegeta".
Logically, it doesn't make sense for the Spirit Bomb to be stronger than Super Saiyan Blue Kaioken Goku as the Spirit Bomb would have to be at a Universal Level if that were the case and the only fighter who contributed sufficient energy to be categorized at a universal level, would be Frieza(Yet, he was in his final form which isn't universal). If you were to consider the SSJG Goku from the Battle of Gods arc at a universal level, you could essentially consider Gohan at his full power to be around that level however, even he wasn't in his Ultimate form when he contributed energy.
So essentially, I believe the theme of the attack was more or less on the lines of, "Since I cannot defeat you myself, this is a combined attack from the whole of Universe 7".
For the sake of plot, they made the attack look extremely powerful as even the Gods of Destruction of every universe was astonished with this level of power.
Although I personally disagree with this line and feel it logically doesn't make sense, as far as the show goes, Whis makes a statement that Kefla's might as a Super Saiyan, rivalled the spirit bomb. Kefla as a Super Saiyan was around the same level as Super Saiyan Blue Goku. Hence, based on that, the spirit bomb should be around that level of power.