How flaps are used, or how an airplane is configured, really depends on which aircraft, what operation, and what maneuvering is being done, as well as how much flap is to be used.
In the B747, we have minimum flap retraction speeds on takeoff, for example, and limit ourselves to a 15 degree bank angle until 10 knots over those speeds. If given a turn during the clean-up portion of a departure, I will accelerate to 10 knots over the bug speed for a given flap setting, before bringing up the flaps, in order to use the full bank angle in the turn. With flaps retracted, I'm limited to a 15 degree bank until I accelerate a little more, and then I can use the full bank.
We don't do a lot of wild maneuvering in the 747, obviously. In the case of the ag airplane previously mentioned, indeed many modern ag aircraft, flap use is a regular thing. Many ag operators will use flap in a turn and take it back out as one rolls out of the turn, especially when "heavy." The flap switch on a number of ag airplanes is on the throttle, so the flaps can be thumbed in or out while maneuvering. The flaps increase stall margin and reduce turn radius by allowing one to pull a little more through the turn. Larger turn radius and higher speeds equate to more time spent turning and less doing work, and one usually tries to minimize time spent in the turns, and maximize time spent applying chemical in the field.
Likewise, I've done a lot of low level work in the mountains doing firefighting, and flap application is very important, there. One large four-engine airplane I flew doing fire work required 1/2 flaps on the drop, and this made a 50 knot difference in the stall speed...very important in mountain gusty conditions.
Retracting flaps in a turn is not unsafe, but should be done sensibly. The notion that one should never retract flaps in a turn is nonsensical; a turn has no bearing on flap retraction. The speed and energy one has at the time, and one's relationship to angle of attack and minimum speeds, of course, is very relevant. If one is too slow to retract flaps in the turn, then one is too slow; either speed up or wait. If one has adequate energy, margin, and speed, then by all means, put them up or down in the turn.
Flap assymetry is flap assymetry, whether in the turn or not.
The airplane doesn't know if it's in a turn. The airplane doesn't know it's altitude. The airplane knows nothing...flaps retracted in a turn, flaps retracted at 5' of altitude...the airplane doesn't know or care where or when. The airplane can't think...and is perfectly happy two seconds from impact if you fly it into the side of a cliff. The determining factor, then, is you. Raising the flaps is perfectly safe in a turn, if you do it properly. Raising the flaps at 5' above the ground is perfectly safe, if done properly.
Policies exist in various aircraft, either given by the manufacturer or by the operator, regarding aircraft configuration, flap retraction, etc, and these generally dictate how the airplane will be operated. This doesn't mean that raising the flaps in a turn is unsafe; it does mean that an operator who improperly raises them in a turn, perhaps while too slow or at too steep a bank angle for a given angle of attack and airspeed, may be doing something unsafe.
Now...there's a difference in the way a B747 is configured with flaps, and an Air Tractor AT-802 is configured with flaps, as well as a difference in the airfoils, flap retraction times and methods, complexities, flap type, etc. With the initial applications of flap in the 747, for example, not only are the trailing edge flaps configured, and not only do they extend aft and down to increase the wing area, but two types of leading edge devices are deployed to increase wing camber and change the angle of attack. The 747 is a swept wing airplane with a very large wing and a very heavy weight capability. It's mission is different, and it's maneuvering tends to be a lot more benign than say, a Thrush S2R or Dromader M18T. Never the less, the same basic principles apply.
If one is too slow, then raising flaps may be a bad idea. Sometimes raising flaps can be used to good effect. On many occasions over the years in light airplanes, I've raised flaps as I touch down, or just before touchdown, in a light airplane on a rough surface. It's always a standard practice in ag airplanes, for me, too. Retracting flaps helps positively set the airplane down and keep it down. Standard for me in the 802 or Dromader, for example, is to put the airplane on the mains, pin the mains with some forward stick, then retract the flaps while holding the tail off, lower the tail to the surface, and pull reverse. Others whom I know tend to go right to reverse with the tail in the air. I don't do that. But retracting flaps ensures that an airplane which was just barely flying with full flaps, is no longer going anywhere, and it sticks to the ground...no more lift. It also allows the tail to come down more quickly, which allows braking and or reverse to be more effective, and safer. Retraction of flaps is a useful short-field technique on soft and rough fields...and in the real world of short-field work, short fields are often rough fields...where props toss rocks into flaps, and where power plants and props can suck up rocks, sand, and gravel, and be damaged.
Likewise, in a light airplane such as the 802 (light being a relative term; at 16,000 lbs, it's a heavy single-engine light airplane, but it's also performance-limited at that weight, which means it's also got a reduced maneuvering margin), I commonly retract flaps just after takeoff...often while still in ground effect, though sometimes I'll carry flaps during the climb-out, too (a little flap seems to help). Very often in the M18 Dromader, I'll have takeoff flaps retracted by the time I reach the end of the runway, and will initiate my turn on course from ground effect, at a low level, while doing the flap retraction. Again, it's a matter of being sensible. One knows, even as a student pilot in a Cessna 150, that simply retracting flaps and doing nothing might lead to a descent back to the ground, a stall, a wider turn, etc...all depending on what one's doing at the time. This is no different in a Cessna 150 than it is in a Boeing 747...despite the obvious disparity in complexity and operation. Operate sensibly, and you can most certainly apply and retract flaps in a turn.
Something that hasn't been addressed is maneuvering while applying flaps. Loading the structure and the airplane can have an effect on the wing and flaps, and any time flaps are applied, a load is applied to all the attach points as well as the wing itself. Rolling into a turn or out of a turn with aileron applied and other turn devices (ala 747) can place a different load on the wing and it's associated components than if one applies the flaps in level flight. This is largely inconsequential, however, if one operates the airplane inside the known and published flight envelope, respecting the speeds, weights, bank angles, etc, that apply to a given airplane.