In most cases they are more of a pain period...not to many boat builders care about servicablity..once rolls out their done !!

Both race cars and boats, its all the same. Some require more patience than others.
A couple of tips from over the years working in tight spots:

1. If the water lines on the bottom of the exhaust maniflods have hoseclamps, change them out to T-bolt clamps. The sharp end of a hoseclamp is nasty on the arms changing plugs.

2. Don't try to do anything in a hurry or on a deadline. You have to be in a patient state of mind, or everything will seem much worse.

3. Invest in some 'flushcut' cutters for shortening the tail off of zipties. The sharp edge left on a ziptie after using regular dykes is sure to draw blood in tight quarters.

4. If you get into a certain area working on the boat, replace all the consumable items while you already have that area apart. As soon as I bought my boat, it got sea pump / water pump / belts / hoses / fuel pump / fuel lines / thermostat / spark plugs and leads / filters etc etc all at once. You don't want to be down in that hole anymore than you have to be, because its even worse when waves are knocking you around. Preventative maintenance becomes your best friend.

5. Accept its part of having non staggered twins. It can be a pain in the azz. So can sitting dead in the water from a hurt engine or drive on a single engined boat. Everybody has their own flavor of what they want to deal with. I prefer the extra expense / time working on my boat for the peace of mind, the performance, and the sound

6. As someone else said, room in front of the engines to climb into the boat makes a huge difference. Was a part of the reason I chose the Fountain over a couple other options.

And, I can't afford a staggered set-up I mean, really, who wouldn't want that?

All depends how good you are and how hard you like to work. I was once lowered into an engine compartment by my ankles to change an impeller on an 80's 30' baja. Gotta be able to trust your friends to pull you back out too!

Point is, if you like the boat you can find a way. If its not worth the work, you should probably have a different boat.

Twins are great except when on the rag then they tag team and smack the pi$$ out of you. On my Velocity to change the plugs I strip down naked smother my body with baby oil and bang the hell out of the twins while my little chinese mechanic changes my plugs.

Very good advice