On the product side, as the poster above said, customers bought VMWare for a reason and will be resistant to move to Citrix. They will likely just migrate to AVD+ Nerdio at their leisure. The products are different enough that the internal savings won't be realized that quickly.
And there would be some concern with the FTC in terms of market share and permission to merge. Given the long-term demise of EUC, the FTC may just say go ahead and strap the two dropping bricks.
Lastly, the immaturity of Citrix leadership to pull this off would be the biggest risk. The only real successful acquisitions have been mostly independent on the product side: GoTo, Netscaler and Sharefile. With VMW EUC, they are serving the same needs, so the internal battles would be glorious to watch but disastrous at the end!
I think Citrix's biggest worry should be some PE buying the VMVEUC at a cost effective price, unlike the boat anchor debt that's on TK's neck. With better freedom to move, and unshackled from VMV's original sales straight-jacket, they could be even more aggressive on the market, partner deeper with MSFT and hasten Citrix's demise...
In a stagnant market, this is one of the worst things can could happen to Citrix.