The issue with using co2 or erbium is highly dependent on the skin type, since Fitzpatrick 4 and above cant use co2 or not preferred, here are some studies showing improvement with erbium in acne scar patients.
These are some studies I saw on a website, so just want to share with the community here:

SCIENTIFIC & CLINICAL RESEARCH LIBRARY:

Laser-Induced Optical Breakdown - Acne Scars Treatment: Pedro Alberto Santos J LA&HA - J Laser Health Acad 2018; 2018(1):CB11 - Read Full Text

Er:YAG Laser for Acne Scars: Ursa FlorijancicJ LA&HA - J Laser Health Acad 2017; 2017(1):CB02 - Read Full Text

Treatment of Active Inflammatory Acne Vulgaris with 1064-nm Nd:YAG Laser: Ruta Ganceviciene, Raimundas Meskauskas, Ausra Berzanskyte, J LA&HA - J Laser Health Acad 2015; 2015(1):S09 - Read Full Text

CASE REPORT: Treatment of Acne Vulgaris with Long-Pulsed 1064 nm Nd:YAG Laser: Robin Sult, J LA&HA - J Laser Health Acad 2014; 2014(1):57-60 - Read Full Text

Combined Er:YAG Laser Acne Scar-Revision Treatment: Yana TargonskayaJ LA&HA - J Laser Health Acad 2013; 2013(2):B02 - Read Full Text

Comparison of Fractional Er:YAG and Carbon Dioxide Lasers in Resurfacing of Atrophic Scars in Asians. Manuskiatti W. Journal of the Laser and Health Academy, Vol. 2011. No. 1.Read Full Text.

Combined Use of Nd:YAG and Er:YAG in Acute Cystic Acne Treatments - A Case Study.
Kakas P. Journal of the Laser and Health Academy, Vol. 2010. No. 1. P. S02.Download PDF | 624 kB

Treatment of punched-out atrophic and rolling acne scars in skin phototypes III, IV, and V with variable square pulse erbium:yttrium-aluminum-garnet laser resurfacing. Wanitphakdeedecha R, Manuskiatti W, Siriphukpong S, Chen TM. 2009.Read Abstract.

Retrospective analysis of non-ablative scar treatment in dark skin types using the sub-millisecond Nd:YAG 1,064nm Laser.Badawi A, Tome MA, Atteya A, Sami N, Morsy IA. 2011.Read Abstract.

Treatment of atrophic facial acne scars with the 1064-nm Q-switched Nd:YAG laser: six-month follow-up study.Friedman PM, Jih MH, Skover GR, Payonk GS, Kimyai-Asadi A, Geronemus RG. Arch Dermatol. 2004. Nov;140(11):1337-41.Read Abstract.