I will start by saying I only have Boeing experience, but plenty of it, including instructing 250hr cadets.
Manual flying is absolutely essential and should be taught and practiced throughout anyone's career. Generally speaking, the biggest threat in manually flying that I see, is not your ability, as you are aware of this before you disconnect (and have hopefully learned to be humble and not overconfident), it is your PM/PNFs ability to handle absolutely everything that could be thrown at them while you are potentially out of the loop. A lot of things happen between 15nm and touchdown, which judged from the OPs original post they either think the PMs job is easy, or just lack of experience. So what happens on a 737...
- PF Calls for Flap 1, PM checks the speed is appropriate, moves the lever, confirms their action, resets the speed bug for F1 speed, confirms their action
- PF Calls for Flap 5, PM (see above)
- ATC calls cleared for the Approach (maybe with vectors), PM replies to ATC, sets a new heading, sets a new altitude, checks the LOC/GS is accurate, arms the APP, confirms their actions
- Established, PM calls what they see, sets the MAA in the MCP, sets the runway heading in the MCP, calls ATC established
- "Have we had cabin secure yet?" PM calls the senior, waits for them to pick up, asks them if secure or not, comes back and informs the PF
- ATC changes you to tower frequency, PM responds, changes the frequency, calls the Tower, replies to the response to the tower
- PF calls for the Gear down F15, PM checks the speed is appropriate, sets the Gear down, sets the F15, arms the speedbrake, confirms their action, resets the MCP speed, confirms their action, runs the initial flow for the landing checklist
- PF calls for landing checklist, PM reads, looks, waits response, (likely points to reduce PF workload)
- PM seats the cabin crew for landing just as ATC clear you to land, so now slightly behind the curve, but catches up
- PF calls for landing flap, PM checks the speed is appropriate, moves the lever, confirms their action, resets the speed bug for final approach speed, confirms their action
- Aircraft reaches landing gate, the PM accesses if the aircraft is stabilized and a safe landing can be made... calls it.
So we are looking at the PM making approximately 40 actions in the last 7 minutes of the flight, averaged out (which it never is) is something every 10 seconds, and at all this time they are MONITORING the PF (its in the name). They are constantly assessing if or not PF is in a good place, in the loop, ahead of the aircraft, aware of what's coming next while making corrections appropriate to the conditions. They are having to second guess the PF as likely the PF is not very vocal (which they should be when hand flying!), calling out trends, let the PM know that they are correcting etc. Remember your MCC.... you are a crew, not a single pilot.
This is all assuming a very capable and experienced PF, sorry to say at 5 months, the chances are the OP does not fall into this category. The number of times I hear in the sim and on the line, while manually flying, potentially FDs off, the PF call in one breath "Gear DN, Flaps 15, Landing Checks". The PF is forgetting that they have change thrust, pitch trim while following the ILS and LOC in a changing config. You can guarantee they are very much unable to be ready for the landing checks in the time it takes me to move the levers, do my flow, and start calling the checks. The PF does need to be aware of their ability as well.... remember a confirm action after a V1 cut, on your first sim, you are giving it everything to track the centre line, climb, maintain rudder and then your asked to look down to verify a fire handle? Same principle, you have to be ready as PF, have everything settled before calling for a checklist (that you could potentially overwhelm yourself by not been prepared to properly verify and check that item).
Going back to why not let PF manipulate the MCP/FCU? How was your first nav trip in a single-engine? When you looked down and left at your map did you inadvertently turn and descend slightly? Not many pilots can say they flew straight and level while map reading, keeping a plog and hand flying in a C152 on day 1. Same in an A320/B737, you would be amazed the deviations created when the PF has manipulated the MCP/FCU. I've seen significant speed decreases, increase, turns, and increased rates of descents (rarely climbs?). For what reason? They've disconnected the Autopilot, the FDs can be seen almost as decoration as long as you are aware and no that you can fly through an FD. They could of all waited 5 secs, and asked the PM once caught up to change the MCP. All but the experienced seemed to of preferred 5kt speed deviations, or V/S increased of 500fpm and then have to dig themselves out of that hole (again more workload for both).
Why you shouldn't do it as PF, and why Boeing and seemingly Airbus both advocate that the PM manipulates the MCP in manual flight...simple really... experience says it is safer and leads to less undesirable states, which is surely what we all want.