The following are the limitation of IEEE 802.11ac specification.
1. Upgradation of Supporting network
The maximum theoritical network is more than 1Gbps. The uplink network for the 802.11ac access points should support that bandwidth. If its not then there will be a traffic bottleneck and your access points will be limited to uplink network’s bandwidth.
2. Forklifting required
Since 802.11ac specification is relatively new, Current 802.11n access points cannot be supported with a software upgrade. So we need to forklift all the access points and wireless adapters to implement the new 802.11ac environment.
3. Upgrades are usually costly
Total cost for 802.11ac upgradation will be much higher than what you have spent for 802.11n upgradation. You may need to replace access points, uplink/backbone network consists of PoE switches, internet firewall/router etc.
4. High procurement costs
Apart from that the cost of new access points and its accessories(MIMO antennas, ICs, or spectrum analyzers) will be initially high since it is relatively new.
5. Backward compatibility with b/g clients
Many customers are implementing 802.11n with 2.4GHz only. (Remember 802.11n is designed for both 2.4GHz and 5GHz). This is to support 802.11b/g clients in their network. Due to this availablity of free channels for channel bonding is very limited. To address this issue 802.11ac is only implemented in 5GHz thus availing channel bonding but losing b/g clients in network. Obvious that only upgrading infra is not enough but to upgrade our personal devices also. (P.S. Cisco, Apple, Netgear, Acer are already giving good products to support 802.11ac specifications, Refer my old blog post about 802.11ac)
6. Less possiblity to escape from Interferences
Larger channel width is required to support high bandwidth. Therefore technology has combined many channels. As a result number of non-overlapping channels has reduced. In a large dense network environment it may be tough to avoid interference. Lets see how cleanair and other interference avoidance systems solves this problem.
7. Not so fast Client adaption
Practically speaking, all of us not gonna upgrade our laptop with 802.11ac adapters. So though the network is a 802.11ac network, it may need to work on 802.11n mostly.
8. Different radio from 802.11n
802.11n used 2.4Ghz and 5Ghz. But 802.11ac is implemented in 5Ghz only. Therefore to support both 802.11n and 802.11ac you need dual band and dual radio access points. That makes procurements costly for customers and positionings difficult for consultants.