now, theres a difference in necros vs wyrms.
necros are sleeper units, considered “trash” since the beginning of wc3. but they are strong actually. frenzy is very good, raise dead is pretty good too, if you can overwhelm the enemy dispel. undead casters were considered trash alltogether, today ppl agree on banshees actually being one of the strongest utility casters.
frostwyrms on the other hand had their time in pro play vs elf, so they got used, and got replaced because they are not worth it. frostwyrms just got played vs elf because elves back in the day decided it was a good idea to play bear dryad vs mass gargs. eventually they just massed dryads or killed off gargs with panda + mass hippos or today they will go mass fearies and there, boom wyrms done.
i used to play them a ton back in the day vs orc, but truth is, hexer lvl3 counters any amounts of frostwyrms without batting an eye. mass wyverns also tear them apart, the same goes for berserkers.
wyrms lose to any form of human anti air, especially dragon hawks and gyros, casters or nuke and wyrms also are really really useless in ud mirror due to gargs, web and destroyers.
they come in so late with such huge resource cost and bring so little to the table, its not worth it. its a bad unit overall.
chims, on the other hand, are significantly cheaper and faster to get, deal massive amounts of splash damage and do have a huge siege attack - also lets not forget elf plays with alot of expos.
gryphons are also a great unit to chime in when you have the resources.
on top of that elf and human have a staff to save those units. if you cannot keep the wyrm alive by hammering in all the death coils you have, its gone.
for what tho? frost nova slows and lich + fiends + dessis deal similar amounts of damage per gold/supply cost.