Look at ag's issue tracker. There are some very critical bugs. You might be impacted by them and not even know it.
As for perf, it's not hard to witness a 10x improvement that you'll actually feel. On my checkout of the Linux kernel:
$ (time rg -wi '\w+(PM_RESUME|LINK_REQ)') | wc -l
real 0.114
user 0.547
sys 0.543
maxmem 29 MB
faults 0
444
$ (time ag -wi '\w+(PM_RESUME|LINK_REQ)') | wc -l
real 0.949
user 6.618
sys 0.805
maxmem 65 MB
faults 0
444
Or even basic queries can have a pretty big difference. In my checkout of the Chromium repository: $ (time rg Openbox) | wc -l
real 0.296
user 1.349
sys 1.950
maxmem 71 MB
faults 0
11
$ (time ag Openbox) | wc -l
real 1.528
user 1.849
sys 8.285
maxmem 29 MB
faults 0
11
Or even more basic. You might search a file that is "too big" for ag: $ time ag '^\w{42}$' full.txt
ERR: Skipping full.txt: pcre_exec() can't handle files larger than 2147483647 bytes.