"what other disadvantages of 150cm gun?"
well...
Based on your 75cm gun... and thinking the difference between my 80cm and 110cm gun. :hmm
its impossible to track the med sea fishes with 150cm gun if you are not exclusively hunting big fish like AJ in open water with boat etc...
very tiring to move around ALL day with it.
very hard to find suitable bands and shaft at that size.
Spear will ruin with the first impact to the rock.Which will happens frequently in typical med sea hunting styles..
150cm gun is hard to travell with probably will not fit in any "normal sized" car...
You will only get 3-4 meter extra range compared to 75 which will serve nothing probably since you will not able to track the fish and shoot the fishes from silly distances by relying on long gun.
You will not able to shoot the fishes in the caves which is the primary hunting style in med sea...
You will have very hard time to load it so you will not want to shoot the fish because the loading process will be so hard.Of course i assume you will be "able" to load it once or twice.
From technical point of view,
The 150cm barell (tube) will be bent, that means accuracy and power loss..
since the spear is long it will be heavy so with single band even with 20-21mm band (probably you will not able to load it with those bands) spear will fly slower and slower after some distance. Thus you will not able to reach those fish lurking far away you mentioned.
You can solve spear speed isssue by putting double bands on it. But by doing this, you will overload the triggger mech., most of the euro trigger mech. may fire itself under heavy load which may cause serious disaster to you or diving buddy. When you put double on it, i can not imagine what will happen to standart euro gun barell. With double bands there will be too much recoil and standart euro guns are not designed to handle (counter balance or counter recoil) such power so your shoot will hit anywhere but not the target :hmm
There are many more diadvantages such lon gun but you have try to see
If you want versatile long range gun that will accompany to your 75 get 110. Seriously those things (spearguns) are not firearms you can not expect to hit the target very far away. You should practice your technique, i did not see or heard anybody whose needs that long gun to get specific fish in med.
If you want long range gun and not thinking of doing cave hunting you can get 106-120 cm double band wood guns from italy. They are good guns and you can find decent info about them if you search the forum but no matter what gun you are using, it does not guarentee the succesfull hunt. Its the diver that matters not gun.