Those type of aircraft get countered very easily by SAMs, like the Imp Chaparral.
They can also proxy the bullpups if they’re good enough, and if you are not careful.
IRCCM missiles, like stingers, can catch people off-guard, especially if you are not careful.
They can be dodged fairly easily, sure but they can become more difficult when you are facing away from the missile, instead of going towards it. They also force you to bleed energy, which means that you may not have enough to dodge the second, especially with delta wing aircraft.
Su-39 at 11.3 can do quite a lot of things very well.
Aircraft? R-60Ms and really good radar (Has TWS, SRC PD, and dogfight ACM mode etc) to find and slave R-73s onto planes during dogfights.
Vikhirs can even be used (if they do not realise) in GRB head-ons, as they have proxy and are quite fast.
They can be slaved onto the radar lock as well.
Not really slow, unlike A-10s, so they can catch up to some things if they are not aware.
They aren’t delta wings, so they have fairly good energy retention.
The flaps are great, allowing them to turn fairly well, especially with landing flaps.
The gun is one of the best in the game for anti-tank and anti-aircraft, at least in my opinion:
Great damage, good firerate, APHE, good velocity, good placement of the gun, doesn’t need to spool up. The only thing lacking that I notice a lot is that it only gets 200 rounds.
SPAA?
Just shoot two KH-29TEs and they’re most likely not going to intercept them on time, with them going mach 2.2 and having a HE warhead rather than HEAT / SAPHE with mavericks.
Ground targets?
16 vikhirs and 2 KH-25MLs (for easy one-shots).
Integrated camera is not too shabby, especially for 11.3.
Radar has TWS GMTI, so you can find moving targets (even through smoke) and shoot them with vikhirs (slaved onto the radar) too.
Helicopters?
16 vikhirs, which have great range, speed, and has proxy fuse - which can slave onto your radar too.
KH-29TEs as well if they are not aware.
There are definitely other great 11.3 CAS vehicles, like the Mirage 2000DR-1, but it’s generally not as multi-purpose as the Su-39, at least for large quanitites of enemies.
It was the best, but it’s now at 0.3 BR higher than any other SPAA (0.7 for the HQ-17, for example), and is outshined by the Pantsir at its own BR (in most cases).