Appointment systems are not immune to politics or external pressures from special interest groups. Most nominating commissions include elected officials and members of the plaintiffs or defense bar. These members are inclined to nominate individuals who are supportive of their interests.
Justices may also become the agents of those who appointed them. For example, the current U.S. Supreme Court, appointed by President Trump, has granted 17 emergency stays (out of 29 applications) to the Trump administration even though the lower courts have held most of them invalid. During the 16 years of the Bush and Obama administrations, the government asked the justices for such emergency relief only eight times while the justices granted half of the applications. A stay from the Supreme Court allowed President Trump to divert military construction funds to build his border wall. Similarly, stays from the Court have allowed the Trump administration to enforce a variety of asylum policies, including the "Remain in Mexico" program and the public-charge rule. As we see from this example, it is possible for a Supreme Court to become politicized under certain leaders.
Therefore, the appointment is not always made based on the qualifications of the candidates. It can still produce a Supreme Court with unsuited and politicized justices.