When a local chapel was started over 40 years ago, the priest likely had different kinds of prior experiences working in the various parts of the Church, other than SSPX, so that was some assurance as to its Catholicity. But SSPX priests now likely have ministry experience only in SSPX.
The 1970s SSPX priest had experience working under Ordinaries, some of whom were orthodox, in the eyes of SSPX. He also had Direction from Archbishop L, who had experience as an Ordinary, who had served in a variety of ministry settings, as did many of the other early leaders. That variety of direct, personal hands on experience benefitted the 1970s chapels. It assured some degree of Catholicity. For a few years.