What is wrong with mead yeasts?
Nothing per se is "wrong" with them, they will make a fair mead, from my own experience giving one a try out of curiosity and then reading multiple threads of others that have used them there is a general consensus on the different strains.
Wyeast:
Sweet mead smack pack....It will leave you with a desert level of sweetness even in a lower gravity must and it is very fickle and very high maintainance. It will need constant daily supervision to finish.
Dry mead smack pack....This seems to have a better reputation, there have been more documented supporters of it and it seems to be able to work a little more independantly with just routine maintainance (step feeding, aeration, etc..)
The white lab tubes are talked about less but it still seems the general opinion is that the sweet is very sweet and the dry is very dry.
the purpose of the smack packs and liquid tubes is convenience, yet it is recommended to make starters with them, while dry yeast works very well with simple rehydration methods. The convenience factor is eliminated.
Then there is the cost factor, one activator (smack pack) or one tube costs as much as 5-7 packets of dry yeast depending on where you buy it.
Consistency is also questionable, how have the tubes/activators been stored? they are effected by temperature spikes much more than the dry yeasts. have they been in a warehouse for awhile, were they shipped on ice packs? have they been in storage for awhile? if you order online do they provide an ice pack or is there an extra charge?
there are just so many more things to consider with the liquid "mead yeasts" that make dry proven wine yeasts that give many more options as far as strains go a more appealing option