I have seen nothing yet that actually shows that SF2 is a deeper, higher skill required, or more complex game than UMK3. Complexity of performing moves is limited to player ability and not actual mechanics of game complexity. I saw a match where two Guiles in the newest incarnation threw over 300 Sonic booms in one 3 round match and only screwed up a couple times, in fact one round was entirely Sonic Boom zoning and no contact was made. The throw system in SF is actually very simple. There are times when you can be thrown, there are times when you can't. That's it. Learning this is key. In MK, you truly have to confuse your opponent to throw them consistently if you are in high level play. Tick throws are tricky, I will say that.
By the way, I am fully versed in the SF/Capcom fighting game franchise, I even know a bit about some of their other games, I am practically a professional at some of the Versus Series games as well.
If we go with just SF2, the answer is flat out no, SF2 is not anywhere near UMK3, and we'll start by just talking about the run button and block button and what they do to change game physics and overall strategy. The run button alone puts UMK3 over SF2. At high level, SF2 is basically a cross screen turtle fest of Guile vs Dhalsim, exactly like MKII is of Mileena vs Jax, cross screen projectile war, instead of erasing, you are avoiding.
When you get to the faster versions with the bosses, you have some more variety and some changed concepts for balancing, but it is still the same game, it's still played the same way.
Now, SSF2 is where it starts evening off, and I would say even Super Turbo is still a tough call. Super moves alone don't really do anything but add damage to critical points, they are just like heavy combos in UMK3, except you don't perform them on your own.
People say the combos aren't predetermined in SF2 series. Yes they are. There is a limit as to what you can do with the system, and certain situations will leave you with a best available combo at all times. You have to perform them differently, but in most situations, D+LKx3, or JK, D+HP, fireball, is not as complicated as Jax's 7 hit combo. Comboing different degrees of hits in SF2 series is extremely limited, it's based on a just frames system where you have to activate the next hit as soon as it is available, and there is a semi canceling system for comboing hits into specials, and it's extremely easy to learn. There are just frames all over the place in UMK3 which make certain juggles possible, there is also no juggling system until SSF2, and even that is terribly limited.
When you get into the alpha series there are certain steps back from Super Turbo, and I would say Alpha 2 approachs the same level of competition and depth. SFA3 is where it unquestionably surpasses UMK3.
SF3 3rd Strike is also unquestionably superior to UMK3, but I would liken 3S to UMK3 in terms of overall achievement and completion of gameplay per franchise, even though of course, more can be done with both.
So in the end, the only games that are definitively more complex than UMK3, are SFA3, and SF3:3S, there are a couple which are debatable like A2 and the other SF3's, and essentially the entire SF2 series is not comparable. The gameplay is very limited.
A and MKA fans.
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