Keith Burnikell

OP, engaging any more-experienced martial artist looking for a favorable outcome is a tall order.

Can Aikido be effective against other martial arts? Yes.
The issue appears to be 'why does it appear to take so much longer to become 'proficient' in aikido?

Many reasons. The two most obvious are repetitions and training mindset.
Repetitions. I'll explain.
Haganah has 16 week rotations. Lot of reps. Krav Maga has similar drills.
How many punches does a boxer throw in a year? How many combos thrown in Karate, TKD, MuayThai? How much uchikomi is done in a Judo class? Assuming that all of the MAs have good instructors it boils down to how quickly the requisite skills are gained and NOT playing the other guy's game. Note I said skills, not techniques.
I believe that the answer lies in lots of quality repetitions. Going through full throws in Aikido (especially projections) eats up a lot of time and exhausts ukes quickly. Not so in other martial arts. So, there's a potential for disparity in the efficiency of gaining knowledge. This can be remedied by stopping short of the actual throw/pin 4 out of 5 times. Then there's the 'talking'. How many times have aikidoka had to sit through 10-15 minute breaks expounding about O'Sensei and other such topics and crucial time is taken away from a 2 hour class. In many Aikido dojos, there's simply far too much talking.

Training mindset:
Go to a boxing gym and look at the training dedication, focus and discipline. They don't hide behind philosophies for why their techniques don't work, they train to fix the issues.

Perhaps that's why dedicated boxers with ~ 3 years under their belt are usually far better at boxing than the usual 3 year aikido practitioners are at aikido. It's not just skill, it's the peripheral aspects. They're fitter and tougher mentally. They learn their sport thoroughly and WILL annihilate you if you stand in front of them. Evading won't help much. They're masters at eating up space and not overcommitting. However, after hanging out with my friend who was a pro-fighter he demonstrated very subtle deflections and parries. I was amazed at how soft they were. In fact, at his level (and he was good) I saw a great similarity to Systema.

I have trained with an Aikidoka that I felt trained in Aikido with the same fire and focus that boxers train. His aikido is impressive. He's a no name to most people on here but I have seen him mix it up with a trained fighter. Aikidoka was 150lbs soaking wet. The guy he fought was 230lbs, highly trained and younger. Result, the big guy was a dish rag when he was done. No aikido principle was broken at any time that I could see. Aikido works.

Story two: I've witnessed an Aikidoka use aikido to stun a proficient MMA practitioner! It did involve the appropriate use of atemi. IMHO, those instructors (no matter the rank) that fail to include atemi waza in the training of their students are doing them a HUGE injustice or poorly trained themselves.

Given enough repetitions, the inefficiencies disappear and you distill your skills down to the fundamental principles. Then you can train for specific opponents but the principles remain the same.

As to the previous comments concerning carrying firearms: I agree, firearms are highly effective but use of a firearm WILL be a life changing event for you. Period. My breaking anyone's arm while controlling him is a lot different to resorting to shoot him in the face in response to a dangerous beat down. In addition, there are many times I am not allowed to have a weapon on my person...e.g. airports. Aikido's really nice to know in such a circumstance.

Summarizing, if Aikidoka trained with the same focus and efficiency as boxers/Muay Thai practitioners their learning curve would be much steeper. I don't believe that such an approach to training would in anyway compromise the philosophical benefits of Aikido.