Pros
alliantgroup does one thing incredibly well: it makes money. In turn, it pays really well. This is a great place to use as a springboard, while making a great salary (at the staff to manager level, you will make more here than you would at either a boutique or a public accounting firm) if you want to specialize in either the R&D tax credit or the 179D deduction. Please keep in mind that these a very niche specialties in the tax field.
Cons
To be fair, alliantgroup is not the worst company in the world. But it certainly isn't the best, and you will have better opportunities elsewhere. In summary, you will work for some of the weirdest and least qualified/professional cowards you will ever meet. The culture is very reminiscent of North Korea or Stalinist Russia where the CEO is worshipped and your peers are too afraid to be honest with themselves and with each other. The culture promotes backstabbing and "loyalty" to the firm. One of the most cowardly things I've ever experienced in my life was being told I was not permitted to maintain a friendship with a colleague who left the firm because they were a "low performer" and maintaining a relationship with them put my loyalty to the firm in question...