can i use nginx instead of Citrix NetScaler for xendesktop

To further Tony's comments, NetScaler Gateway functionality and XenDesktop compliment each other.  There are a few use cases where other ADCs are use, but the vast majority of XenDesktop implementations utilize NetScaler for remote access.  

NetScaler Gateway is basically an ICA/HDX proxy that provides security and data flow.   That functionality is a subset of the full ADC.  So, the comparison that you're seeing is not full apples to apples.

Because Citrix is the common denominator for both XenDesktop and NetScaler, it serves to reason that they work best together.  Yes, Citrix does charge what could be perceived as a premium for it, but the integration and support will minimize headaches.

I agree with the comments above that you should compare with VPX as it is a virtual appliance like NGinx and the pricing is favorable. Also NetScaler has support for ICA and will give you the best performance for XenDesktop. Here is a further comparison of the capabilities of NetScaler vs. NGinx.

Layer 4 Features
NGINX Limitations
DDoS capabilities are limited and configuration requires in-depth technical knowledge
Limited load balancing options
NGNIX support for custom monitor is not as complete as NetScaler

NetScaler Differentiators
DDoS is inbuilt and lot of features are there such as Surge Protection, etc.
Load balancing options include least response time, custom load monitor (SNMP)
Custom monitor options available for backend servers

Layer 7 Features  
NGINX Limitations
NGINX does not have any AAA functionality
No native proxy for TDS protocol for DB load balancing
No Enterprise VPN capabilities
WAF, GSLB, FEO are not supported

NetScaler Differentiators
Complete AAA functionality with nFactor and supports SSO
SQL DataStream native LB for MySQL, MS SQL, and Oracle
NetScaler Gateway Enterprise VPN is available
WAF, GSLB, FEO are supported

Manageability
NGINX Limitations
NGINX GUI for Plus only provides alerting and basic visibility
Configuration is stored in config file and it is difficult to maintain
Configuring HA is complex and requires keep alive module
Troubleshooting is difficult
No Analytics

NetScaler Differentiators
GUI with complete functionality and ease of use
NetScaler MAS provides single console for managing multiple NetScaler Instances
NetScaler MAS has configuration manager/audit/advisor
NetScaler GUI/CLI available for configuration management
HA configuration and maintenance is simple
NetScaler MAS has analytics for web/cache/gateway and troubleshooting application performance is lot easier

We have a free trial version of VPX called VPX Express that you can download from this link. You will need to create an account and login before you will have access.

https://www.citrix.com/downloads/netscaler-adc/virtual-appliances/netscaler-vpx-express.html

Nice comparison, Michael.