How is life as a 91F ?

It all depends on the type of unit you’re in. Some units that are heavy on artillery can give you quite a large workload of things to work on. The rifles and machine guns are all pretty quick.
Typically, we had a very light workload for the most part unless they were certain training exercises going on or were preparing for a deployment.

As for cross Training, there are frequent opportunities in the army to cross train.

The training for the job is very basic and the most important part is learning how to read a manual. What you do learn by taking apart some equipment is basically worthless. You will receive all worthwhile training when you finally get your unit and watch others. I never had a problem with my coworkers in the shop. Just like anywhere else, some were very proficient and some were challenged.

Keep in mind, information I’m giving you is 15 to 20 years old so take it for what it’s worth. Good luck in your career choice, whatever it may be.

From the Guard side of the Army it's a very laid back MOS in my experience. When services come up you will perform inspections and maintenance to ensure the unit's weapons are up to standard. The job and training can be very repetitive with disassembling, inspecting, and reassembling the same type of weapon until all of them have been serviced or the end of the work day. When I first joined my unit after training in 2015 till I returned from deployment in 2019 I was constantly out at the motor pool cross training as a wheeled vehicle mechanic when I wasn't in the vault servicing weapons. Now I work closely with supply although I haven't cross trained with them yet. The best part of the job is being able to be a subject matter expert on all weapon systems that your unit will use. I'm working on stepping up into leading marksmanship training at my unit currently due to my knowledge of the weapons and marksmanship. The worst part of the job is that the chance for promotion in the Guard is difficult at best. There are only a handful of other E5 91F positions in my state other than mine and even fewer E6 positions.

Not trying to brush off your questions but, like any MOS, the difficulty of the training depends almost completely on you and your ability to learn. The day to day and ability to cross train will depend on your unit. I instructed this MOS for 9 years. Most of the Soldiers I stayed in contact with enjoy the job. I don’t think it’s a difficult MOD but it does require intelligence and attention to detail. You are repairing and possibly certifying the weapons your units will take into battle.