Just wondering what the feelings are on this. Would you use a shop that has completed all the required factory schools and been doing engines for many years but is not an faa repair shop for overhauls and repairs.
Any engine shop should be evaluated on it's own merits.
There are many "not FAA certified" shops that do great work. On the other side of the coin the certified shops got their certification for a reason.
Best evaluation is word of mouth. Talk with people who have had work done there.