As with most things related to Traditional Catholicism, there are more questions than answers.
In the end, it is up to each individual to decide if he should attend this or that mass or none at all.
Some people believe that the indult is ok. Some people think that the SSPX is ok. Some people think that Una cuм masses are mortal sin. And so on.
Let's pray that God will be merciful with everyone. I believe that He will not punish people of any (Tradionalist) position, if they are sincere about what they are doing.