What are the ups downsides of mecanum wheels
They have a ton of rotational inertia, and accelerate slowly as a result. The upside is that you can get holonomic motion without having to mount things at diagonal angles on the chassis, with all of the regular pros and cons of a holonomic drive.
Bulky and there is a lot of friction.
Would it work well with a 4 motor v5 chassis
VERY difficult to climb, but awesome for adjusting to line up your shot. Personally we use them on our robot and it is very useful because it is critical to line up good for our double catapult. We had to get creative to climb. And again it is pretty bulky.
Do you focus on flags or caps?
Are the turns any slower and how much slower does the mecanum wheels make your drive
Flags
We have a double catapult
Turning is not bad
And it isn’t super slow especially with 4 motor drive
Since you have a double catapult I would recommend them but you should have some raised omniwheels in the middle
Ok thanks
but Again it won’t climb very well on its own you will need an angled bar to push your robot up until the lower half of the wheel will be touching the platform to pull it up.
Oh another pro I forgot to mention, although it is hard to get on the platform it is easy to adjust after you’re on it to avoid falling.
I have seen robots work quite well with V5 mecanum actually. You could give mecanums a shot a give your input regarding as to what you think about mecanums 
We have mecanums and they work very well. Omni wheels climb better, but we can still climb and if somebody tries to push you off from the side you can strafe and keep them back. You can see our reveal that we recently posted and see how we work with them.
@dchoi77 Mecanum Wheels provide the ability to shuffle and slide horizontally with a strafe, and is ill equipped to turn as well as other options. I am not sure how the mecanums compare in terms of climbing the platforms, but it would seem that less grip would be applied with them compared to the regular 4" wheels.
We have a Mecanum Drivetrain and our bot doesn’t struggle to climb. Once in awhile the bot will slide sideways while climbing but there is no other assistance for it to climb just straight Mecanum
Do you have a picture of your Mecanum Drivetrain.